Everglades National Park

Long ago, this land was under the sea.

Now it is very flat. Water flows slowly south across it, like a wide, shallow river. Grass grows in the water.

UNESCO explains that the Everglades is a vast, nearly flat old seabed that was under water at the end of the last Ice Age. The National Park Service says the Everglades formed about 5,000 years ago on top of porous limestone, a rock full of tiny holes.

The land is so flat that a few inches of height decide what grows where, the park service says. Fresh water flows slowly into the park from wetlands upstream, to the north. Near the coast, fresh water meets salt water, and there mangrove forests grow.

UNESCO's geological criterion describes the Everglades as a vast, nearly flat seabed submerged at the end of the last Ice Age, whose limestone substrate is one of the most active areas of modern carbonate sedimentation. The National Park Service says the Everglades formed about 5,000 years ago atop porous limestone bedrock, that elevation differences of only a few inches shape which ecosystems develop, and that salt shapes the coastal zone.

The park lies at the southern, downstream end of a 4,660,000 ha watershed covering the southern third of Florida, with state-managed Water Conservation Areas just upstream to the north. Its ecosystems range from freshwater marshes and sloughs to tropical hardwood hammocks, pine rocklands, mangrove forests, salt marshes and seagrass beds. Fire is a natural part of the system: the park service notes that some plant communities are fire-dependent.

Alligators and crocodiles both live in the Everglades. They like to lie in the warm sun.

Tall wading birds walk in the water. Manatees swim in the bays. A rare big cat, the Florida panther, lives here too.

Both alligators and crocodiles live in the Everglades. You may see them lying still by a trail, warming up in the sun: reptiles get their heat from their surroundings, the park service explains.

UNESCO says the park shelters more than 20 rare, endangered and threatened species, including the Florida panther, the snail kite, the American crocodile and the manatee, and gives food and nesting places to more than 400 kinds of birds. It has the most important breeding grounds in North America for wading birds, such as the roseate spoonbill.

UNESCO highlights an unbroken food chain from algal associations to top predators such as the alligator, crocodile and Florida panther, and a mix of subtropical and temperate species found nowhere else in the United States. The park provides refuge for more than 20 rare, endangered and threatened species, including the Florida panther, snail kite, alligator, crocodile and manatee, and habitat for over 400 bird species, with North America's most significant wading-bird breeding grounds.

Britannica lists over 40 mammal species, about 300 fish species and birds from the roseate spoonbill to the bald eagle, and notes that Florida panther sightings are exceedingly rare, with estimates of fewer than 250 left in the wild. UNESCO warns that wading-bird populations in the northern wetlands are estimated at just 10% of pre-drainage levels.

Many people live near the Everglades, in cities like Miami.

Long ago, people moved much of the water away for towns and farms. That left the park thirsty. Now people are working to send more water back.

Between 1900 and 1970, people built drains and water-control systems that moved water away from the Everglades, for flood protection and water supply. UNESCO says these changes to the water are the biggest environmental threat to the park: much less fresh water now reaches it, and the timing and quality of the water have changed.

The park service says that Everglades restoration will restore the water's flow. The park itself contains only about 20 percent of the original Everglades, but it still has examples of all its original habitats.

UNESCO identifies water management as the largest environmental threat: inflows at the park's northern boundary are believed to be about 60% lower than pre-drainage estimates, with altered timing, distribution and water quality. Reduced inflows have cost deep-water slough habitat; agricultural nutrients have changed marsh plants and reduced periphyton; and higher salinity in Florida Bay has changed seagrass, reduced sportfish and spread algal blooms. Organic soils were lost to fire and oxidation as the drainage system was built between 1900 and 1970.

The park lies beside a growing metropolitan area; Miami-Dade County maintains an urban development boundary as a buffer to the east, and the park is surrounded by other protected lands including Big Cypress National Preserve and Biscayne National Park. The National Park Service states that Everglades restoration will restore water flow. UNESCO also lists hurricanes, invasive species and ocean acidification as pressures.

Stay far away from alligators and crocodiles, even if they look asleep.

Hold a grown-up's hand near the water. Never feed any wild animals. Don't swim in the canals or ponds.

The National Park Service asks visitors to stay 15 to 20 feet (4.5 to 6 meters) away from wildlife and from the shoreline. An alligator or crocodile lying still is not hurt or fake: it may be warming in the sun, and it can move fast. Keep small children and pets away from the water's edge.

Swimming and snorkeling are not allowed in the park's canals, ponds, freshwater lakes, marked channels or boat basins. Feeding or bothering alligators, crocodiles and other wildlife is a crime, because fed animals can become dangerous. Drones are not allowed in the park.

Park guidance is specific. Keep 15-20 feet (4.5-6 m) from wildlife and shorelines; never assume an animal is hurt, dead or fake (the only animal statue is a Florida panther outside the Ernest F. Coe Visitor Center); prevent small children and pets from approaching shorelines or basking alligators and crocodiles; and stay alert on beaches, boardwalks, trails and docks. Swimming and snorkeling are prohibited in canals, ponds, freshwater lakes, marked channels and boat basins.

Feeding or harassing alligators, crocodiles or other wildlife is a criminal offense, because animals fed by people associate them with food and can become aggressive. Pets are restricted to parking lots and frontcountry campgrounds on a leash, and drones and other unmanned aircraft are not allowed. Pythons, though alarming, are rarely a risk to people; USGS notes that known constrictor fatalities in the United States have involved captive snakes.