What If? Lab
What if the sea were as clear as glass?
Some seas look blue, some look green and some look muddy. What if the whole sea were as clear as glass? Let's look with Olly!
Sunlight fades as it goes deeper into the sea, and below about 200 meters there is hardly any. Tiny drifting plants and bits of mud also change how green, blue or murky the water looks. Imagine the sea perfectly clear, then follow how deep light would go, what it would mean for seaweed forests, and what gives the sea its colors.
Seawater absorbs red light first, so the open ocean looks blue; phytoplankton and suspended sediment add green, brown or milky tints and shorten how far light penetrates. Significant light rarely reaches below 200 m, and none below about 1,000 m (NOAA). This thought experiment removes particles from the sea and asks what clarity would and would not change.
The big question
Look into a glass of water. You can see right through it!
But the sea is often blue, green or brown. What if the sea were as clear as glass?
A glass of water looks perfectly clear. But stand on a pier and look down, and the sea might look deep blue, bright green or muddy brown, and you often can't see the bottom.
So here is a thought experiment: what if the whole sea were as clear as glass? Could sunlight reach all the way down? Would seaweed grow everywhere? And what makes the sea those colors in the first place? Pick a path and find out.
Water clarity depends on how much light is absorbed and scattered as it passes through. Pure water absorbs some wavelengths more than others, and particles such as phytoplankton and sediment absorb and scatter more.
This thought experiment strips the particles out of the ocean. It asks which features of the sea come from the water itself, and which come from what floats in it, and what a particle-free sea would mean for light, plant life and the food web.
Explore the idea
Make a prediction
If the sea were as clear as glass, would sunlight reach the very bottom of the deep sea?
If all the mud and tiny plants were taken out of the sea, would sunlight reach the floor of the deep ocean?
With all suspended particles removed, would sunlight reach the abyssal seafloor?
- No, the deep sea would still be dark
- Yes, it would be bright all the way down
- The sea would glow red
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Established fact
Sunlight fades as it goes deeper into the sea.
There's rarely much sunlight below 200 meters, and none below about 1,000 meters.
Significant light rarely penetrates beyond 200 m, and none deeper than 3,280 ft (NOAA).
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An idea scientists discuss
A glass-clear sea is pretend.
A sea with no mud or plankton is a thought experiment; what would change is reasoned from how light and water work today.
Outcomes of a particle-free ocean are reasoned from present optics and ecology, not measured.
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Established fact
Sunlight only goes so deep. Deep down, the sea is dark.
Light can sometimes be detected 1,000 meters down, but there's rarely much light below 200 meters.
Light may be detected to about 1,000 m, but there is rarely significant light beyond 200 m; the upper 200 m is the euphotic zone (NOAA).
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Established fact
Most of the ocean is dark all the time.
Most of the ocean is completely dark: hardly any light gets deeper than 656 feet (200 meters), and none gets deeper than 3,280 feet (1,000 meters).
Hardly any light penetrates deeper than 656 ft and none deeper than 3,280 ft (NOAA, Why is the ocean blue?).
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Established fact
Most fishing happens in the bright top part of the sea.
The sunlight zone, the top 200 meters, is where most of the world's commercial fishing happens.
The euphotic zone contains the vast majority of commercial fisheries (NOAA).
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An idea scientists discuss
In a glass-clear sea, light might go a little deeper, but not all the way.
Without mud or tiny plants, sunlight might reach a little deeper, especially near coasts, but the deep sea would probably still be dark.
A particle-free ocean might extend the euphotic zone most where water is now turbid or productive; absorption by water itself would still limit depth.
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Established fact
Kelp needs light, so it grows in shallow water.
Because kelp needs light to make food, kelp forests are rarely found deeper than about 49 to 131 feet.
Kelp forests form in shallow open waters and are rarely found deeper than 49 to 131 ft because of their dependence on light (NOAA).
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Established fact
Kelp forests are homes for lots of sea animals.
Kelp forests give food and shelter to thousands of kinds of fish, invertebrates and marine mammals.
Kelp canopies provide food and shelter for thousands of fish, invertebrate and marine mammal species, and generally occur in cold, nutrient-rich waters (NOAA).
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An idea scientists discuss
In clearer water, seaweed forests might grow a little deeper.
In clearer water, kelp might be able to grow a bit deeper, but it also needs cool water full of nutrients, which a glass-clear sea might lack.
Greater clarity might lower kelp's depth limit, but kelp also needs cold, nutrient-rich water; a sea cleared of plankton might be nutrient-poor, so the net effect is uncertain.
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Established fact
Water soaks up red light. That makes the sea look blue.
The ocean is blue because water absorbs the red colors in sunlight and leaves the blue ones. Light bouncing off floating sediment and particles can make it look green, red or other colors.
Water absorbs the red part of the spectrum, leaving blue; floating sediments and particles add green, red or other hues (NOAA).
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Established fact
Tiny sea plants can turn the water green.
Blooms of phytoplankton can turn water greenish, reddish or brownish, and some types make it milky white or bright blue.
Phytoplankton blooms tint water greenish, reddish or brownish; coccolithophore scales color it milky white or bright blue; scientists use these color changes to estimate chlorophyll (NASA).
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Established fact
The clearest blue seas have few tiny plants.
Tiny drifting plants are scarce in the clear middle of the big oceans, where nutrients are low, and grow best along coasts, at the equator and in cool seas near the poles.
Phytoplankton are most abundant at high latitudes, in equatorial upwelling and near coasts, and scarce in remote gyres where nutrients are low (NASA).
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An idea scientists discuss
A glass-clear sea might have much less food for sea animals.
A perfectly clear sea would have no phytoplankton, and they are the base of the ocean food web, so it might have far less food for fish and whales.
Since phytoplankton are the ocean's primary producers, a particle-free sea might support far less life; clarity and productivity tend to trade off.
Light in our sea
Near the top of the sea, it is bright and sunny.
Deeper down, it gets darker and darker. Very deep down, it is always dark.
NOAA, the U.S. ocean agency, explains that sunlight can sometimes be detected as deep as 1,000 meters, but there's rarely much light below 200 meters. The top 200 meters is called the sunlight zone, and it's where most of the world's fishing happens.
Below that, the water gets dimmer and dimmer. Most of the ocean is completely dark. That's because water acts like a filter: it soaks up some colors of sunlight as the light goes deeper.
NOAA divides the ocean into zones by light. The euphotic, or sunlight, zone is the upper 200 m and holds the vast majority of commercial fisheries. Light may be detected to about 1,000 m under the right conditions, but there is rarely significant light below 200 m.
Most of the ocean is therefore permanently dark: hardly any light penetrates deeper than 656 feet, and none deeper than 3,280 feet (NOAA). The depth of the sunlit layer varies from place to place with how much the water absorbs and scatters light.
Imagine a glass-clear sea
Let's pretend we take every speck out of the sea.
No mud and no tiny plants. Would sunlight go deeper? Would it still look blue?
Imagine taking every speck of mud and every tiny drifting plant out of the sea. Sunlight wouldn't bounce off bits or get soaked up by them, so it might reach a little deeper, and you might see farther down near the coast.
But the water itself still soaks up red light, so the deep sea would still look blue and the very deep sea would still be dark. And without tiny drifting plants, there would be much less food for the animals that eat them. Those are ideas to test as you explore.
Removing particles would remove the scattering and absorption caused by phytoplankton and sediment, so light might penetrate somewhat farther, especially in coastal and productive waters where particles are abundant.
The water would not become transparent to all depths: water itself absorbs red wavelengths, which is why the open ocean is blue, and NOAA reports no light below about 1,000 m even in real seawater under the right conditions. A sea without phytoplankton would also lose the primary producers at the base of the marine food web.
Why the sea has colors
Water soaks up the red part of sunlight. The blue part is left, so the sea looks blue.
Tiny green plants in the water can make the sea look green.
Sunlight is made of many colors. NOAA explains that water soaks up the red colors first, leaving the blue ones for us to see. That's why deep, clear ocean water looks blue.
The sea can look other colors too. Phytoplankton, tiny drifting plants, contain green chlorophyll, and NASA says big blooms of them can turn water greenish, reddish or brownish. Some have chalky scales that turn the water milky or bright blue. Light bouncing off floating mud and sand changes the color as well.
Water preferentially absorbs the red end of the visible spectrum, so transmitted and scattered light is dominated by blue (NOAA). Particles modify this: phytoplankton contain chlorophyll, and NASA notes blooms can turn water greenish, reddish or brownish, while the chalky scales of coccolithophores make it milky white or bright blue. Suspended sediment adds further hues.
Scientists exploit these color shifts: satellite measurements of ocean color are used to estimate chlorophyll concentration and phytoplankton biomass across the globe.
Clear seas and green seas
Some of the clearest, bluest seas have few tiny plants.
Greener seas often have lots of them, and lots of food for sea animals.
Clear blue water is beautiful, but NASA's maps show that tiny drifting plants are scarce in the clear middle of the big oceans, where there are few nutrients. They grow best along coasts, around the equator and in cooler seas near the poles, where winds and currents bring nutrients up from the deep.
People measure water clarity with a simple tool called a Secchi disk: a black-and-white disk lowered on a rope until it disappears. Scientists have used it for over a century.
NASA: phytoplankton are most abundant at high latitudes, in equatorial upwelling zones and near coastlines, where winds drive nutrient-rich deep water to the surface, and scarce in remote ocean gyres because nutrients are low. Clear, deep-blue water and high productivity rarely go together.
Water clarity is commonly measured with a Secchi disk, lowered until it vanishes; lake researchers have used it for more than a century (Cary Institute). Satellites now estimate chlorophyll from ocean color worldwide.
Amazing facts
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Kelp can grow very fast.
In ideal conditions, kelp can grow up to 18 inches in a single day.
NOAA reports kelp can grow up to 18 inches per day in ideal conditions.
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Blooms of tiny sea plants can be seen from space.
Phytoplankton blooms can cover hundreds of square kilometers and are easy to see in satellite pictures.
Ocean blooms may cover hundreds of square kilometers and are readily visible in satellite images (NASA).
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You can make a tool to measure how clear water is.
A Secchi disk, a black-and-white disk lowered until it disappears, has been used to measure water clarity for over a century, and you can make one from a plastic lid.
The Secchi disk has served lake researchers as a clarity index for more than a century (Cary Institute).
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Some tiny sea plants make the water look milky.
Coccolithophores, tiny plankton covered in chalky scales, can make seawater look milky white or bright blue.
Coccolithophore blooms color water milky white or bright blue because of their chalky scales (NASA).
Words to know
- sunlight zone
- the top layer of the ocean, about 200 meters deep, where most sunlight reaches
- phytoplankton
- tiny drifting plant-like living things that make food from sunlight
- chlorophyll
- the green substance plants use to capture sunlight
- kelp
- a very large brown seaweed that grows in underwater forests
- sediment
- bits of sand, mud and other material carried in water
- Secchi disk
- a black-and-white disk lowered into water to measure how clear it is
Discovery challenge
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Why does the sea look blue?
Why does deep, clear ocean water look blue?
Clear ocean water appears blue because water…
- Water soaks up red light
- The sky paints it blue
- Blue fish color it
Show the answer
Water soaks up red light
Water soaks up red light, so blue is left.
Water absorbs the red part of sunlight, leaving blue for us to see.
…absorbs red wavelengths preferentially, leaving blue (NOAA).
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What can make the sea look green?
What can make seawater look green?
A greenish tint in coastal water is often caused by…
- Lots of phytoplankton
- Cold weather
- Deep water
Show the answer
Lots of phytoplankton
Lots of tiny green plants called phytoplankton.
Lots of phytoplankton, which contain green chlorophyll.
…abundant phytoplankton containing chlorophyll (NASA).
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Where does kelp grow?
Why do kelp forests grow only in fairly shallow water?
Kelp forests rarely occur deeper than 49 to 131 feet because…
- They need sunlight
- They are afraid of fish
- Deep water is too salty
Show the answer
They need sunlight
Kelp needs sunlight, which is only near the top.
Kelp needs light to make food, and light fades with depth.
…they depend on light for photosynthesis (NOAA).
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Below how deep is the sea always dark?
According to NOAA, below about what depth is there no sunlight at all?
NOAA reports no light penetrates deeper than about…
- 1,000 meters
- 10 meters
- 50 meters
Show the answer
1,000 meters
About 1,000 meters. That's very deep!
About 1,000 meters (3,280 feet); there's rarely much light below 200.
…3,280 ft (about 1,000 m).
Experiments to try
- Watch What If? Lab What if whales returned to every ocean? Hunting in the last century took away most of the world's great whales, and since a 1985 ban on commercial whaling many are growing in number again. Whale poop feeds the tiny drifting plants at the base of the ocean food web, and whale bodies carry carbon to the deep sea. Imagine whales back in every ocean and follow what they might do.
- Watch What If? Lab What if sea turtles lost their way? Sea turtles cross oceans and return to the coast where they hatched, using Earth's magnetic field like a map. Baby turtles find the sea by heading for the brightest horizon. Imagine them losing their way: follow magnetic maps, bright beach lights, and how people help hatchlings.
- Watch What If? Lab What if you could dive to the deepest trench? The Challenger Deep, at the bottom of the Mariana Trench, is about 11 kilometers down: deeper than Mount Everest is tall. Imagine diving there. Discover how strong the water's squeeze becomes, what lives in the dark, and the explorers who have already made the trip.
- Watch What If? Lab What if coral reefs could be regrown? Scientists grow broken pieces of coral in underwater nurseries, then plant them back on damaged reefs. Follow how coral gardens work, how scientists search for corals that cope with heat, and what happens when fish come back to a restored reef.
Explore next
- Watch What If? Lab What if whales returned to every ocean? Hunting in the last century took away most of the world's great whales, and since a 1985 ban on commercial whaling many are growing in number again. Whale poop feeds the tiny drifting plants at the base of the ocean food web, and whale bodies carry carbon to the deep sea. Imagine whales back in every ocean and follow what they might do.
- Watch What If? Lab What if sea turtles lost their way? Sea turtles cross oceans and return to the coast where they hatched, using Earth's magnetic field like a map. Baby turtles find the sea by heading for the brightest horizon. Imagine them losing their way: follow magnetic maps, bright beach lights, and how people help hatchlings.
- Watch What If? Lab What if you could dive to the deepest trench? The Challenger Deep, at the bottom of the Mariana Trench, is about 11 kilometers down: deeper than Mount Everest is tall. Imagine diving there. Discover how strong the water's squeeze becomes, what lives in the dark, and the explorers who have already made the trip.
- Watch What If? Lab What if coral reefs could be regrown? Scientists grow broken pieces of coral in underwater nurseries, then plant them back on damaged reefs. Follow how coral gardens work, how scientists search for corals that cope with heat, and what happens when fish come back to a restored reef.
- Watch Geography Ocean zones from sunlight to midnight Scientists divide the ocean into layers called zones. Sunlight reaches only the top 200 meters. Below that is the dim twilight zone, then the pitch-dark midnight zone, the abyss and, deepest of all, the trenches of the hadal zone. The deeper you go, the darker, colder and more squeezed by water it gets.
- Watch Try it What If? Lab What if a coast were made of hard rock? Waves wear coasts away, but not every coast wears at the same speed. In our model, a bare sandy island facing ordinary waves keeps about 72 percent of its land after 50 model years. Make the same island of hard rock and it keeps about 99 percent. That is why hard rock is often left sticking out to sea as headlands.
- Watch Try it What If? Lab What if all the sea ice melted? Sea ice floats, so it is already pushing aside about as much water as it will make when it melts, like an ice cube in a glass. In our model, melting all the Arctic's springtime sea ice raises the sea by only about 2 millimeters. The same ice melting on land would raise it about 7 centimeters.
- Watch Try it What If? Lab What if an ocean food web changed? Antarctic krill are small shrimp-like animals, but they feed a huge number of bigger ones. Take krill out of our model's Southern Ocean food web and all seven other species are affected: every animal in it loses all its food, from silverfish to orcas.
For parents & teachers
What children will learn
- Sunlight fades with depth, and most of the ocean is dark.
- Water absorbs red light, so clear ocean water looks blue; phytoplankton and sediment change its color.
- Seaweed such as kelp needs light, so it grows only in shallow water.
- Clear, nutrient-poor seas often support less life than greener, nutrient-rich ones.
Recommended ages: 7 to 17 · About 15 minutes
Talk about it
- Why might a muddy or green sea be full of life while a clear blue one has less?
- What colors have you seen the sea or a lake turn, and what might have caused them?
- Would a perfectly clear sea be a good thing for fish? Why or why not?
Try at home or in class
- Make a mini Secchi disk from a plastic lid colored in black and white quarters, tie it to a string, and lower it into a bucket of clear water, then water with a spoonful of soil stirred in. Where does it disappear?
- Shine a flashlight through a tall clear glass of water, then add a few drops of milk and look from the side at how the beam fades and scatters.
Key vocabulary: sunlight zone, phytoplankton, chlorophyll, kelp, sediment, Secchi disk
Where these facts come from
- How far does light travel in the ocean? , NOAA National Ocean Service
- Why is the ocean blue? , NOAA National Ocean Service
- What is a kelp forest? , NOAA National Ocean Service
- What are Phytoplankton? , NASA Earth Observatory
- Measuring lake health: Secchi disk how-to , Cary Institute of Ecosystem Studies
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.