What if Earth had no mountains?

Mountains are the tallest places on land. Some have snow on top.

What if there were no mountains at all? Would it matter?

Mountains look like they just sit there. But they are busy: they change the weather around them, collect snow and rain, and give rise to rivers.

So what would happen if Earth had no mountains at all, just gentle plains everywhere? In this thought experiment you can follow three paths: what happens to wind and rain, where rivers would start, and what happens to the plants and animals that live up high.

Topography shapes climate and hydrology. The UN notes that mountains provide freshwater for everyday life to half of humanity and host about half of the world's biodiversity hotspots.

This thought experiment asks what might change on a planet with no mountains. It cannot say how such a planet would form; it reasons from what mountains do today, keeping established facts and ideas clearly apart.

When wet wind meets a mountain, the air goes up. It gets cold and makes clouds and rain.

The other side of the mountain stays dry.

When moving air meets a mountain range, it has to rise. National Geographic explains that rising air expands and cools, and cool air can't hold as much moisture, so clouds form and drop rain and snow on that side. Once the air crosses the top and sinks down the other side, it warms and the clouds fade away.

That dry side is called a rain shadow. Death Valley, in California and Nevada, is so hot and dry partly because it sits in the rain shadow of the Sierra Nevada mountains.

Orographic lift cools air as it expands at lower pressure; condensation and precipitation follow on the windward slope. Descending air on the lee side warms and dries, producing a rain shadow. National Geographic cites Death Valley (Sierra Nevada) and the Tibetan Plateau (Himalaya) as examples.

In Rocky Mountain National Park, the NPS notes that prevailing winds drop most of their moisture on the western slope, leaving the eastern slope relatively dry, and that the Continental Divide splits the park's waters between the Colorado River and the Mississippi drainage.

Let's pretend all the mountains are gone. The land is flat everywhere.

Rain might fall more evenly. Some dry places might get wetter. But where would rivers start?

Now imagine every mountain flattened into plains. With nothing to push the air upward, the extra rain on mountain slopes might not fall there, and dry rain-shadow deserts might get more rain instead. Weather might spread out more evenly.

Rivers might change most. In places such as the western United States, mountain snow stores winter water and lets it out into rivers in the warmer months. On a flat world they might start in lowlands and flow slowly. And animals that live only up high might have nowhere to go. These are ideas, not facts: no one can flatten Earth to check.

Without orographic lift, windward precipitation maxima might weaken and rain shadows might shrink, so rainfall could be distributed more evenly, though large-scale circulation might still create wet and dry belts. Mountain snowpack, which USGS describes as a frozen reservoir storing cool-season precipitation and releasing it in the warm season in regions such as the western United States, might largely disappear, changing river timing.

Cold, high-altitude habitats, with the species that depend on them, might vanish. These are reasoned ideas; the climate of a mountainless Earth cannot be predicted by a thought experiment.

Mountains catch rain and snow. The water flows down and makes rivers.

Many kinds of animals and plants live on mountains.

Mountains catch a lot of rain and snow. USGS explains that in some regions, such as the western United States, mountain snow acts like a frozen reservoir, storing winter water and releasing it into rivers in the warm season. The UN says mountains provide fresh water for everyday life to half of all people. The Food and Agriculture Organization of the UN (FAO) says mountains help regulate the flow of water to the places downstream.

Mountains are also full of life. FAO says they support about a quarter of all the different kinds of living things on land, and the UN says they hold about half of the world's biodiversity hotspots.

Mountains act as water stores and regulators: FAO notes that 70 percent of Colombia's freshwater begins in its mountain rangelands, and the UN that mountains supply freshwater to half of humanity. They are home to 15 percent of the world's population.

Biologically, FAO credits mountains with about one-quarter of terrestrial biodiversity, and with being where sheep, goats, yaks, alpacas and llamas originated or were diversified.

The little pika lives high in the Rocky Mountains, among the rocks. It likes the cool air.

People look after mountains so their water and animals stay healthy.

In Rocky Mountain National Park, the waters on the west side of the mountains form the beginning of the Colorado River. High above the trees lives the American pika, a small animal that gathers plants into a store of food for winter. The National Park Service says pikas are sensitive to summer heat and monitors them as the climate changes.

Every year on December 11, International Mountain Day reminds people how important mountains are for water, food and wildlife.

The NPS treats the American pika as an indicator species: abundant above treeline in Rocky Mountain National Park but sensitive to summer heat and to a lack of insulating winter snow; hotter lower elevations already limit movement between populations.

The UN General Assembly designated 11 December as International Mountain Day, observed since 2003, which the FAO leads. The 2025 theme was glaciers and mountain water; the 2026 theme is mountain pastoralists (FAO).