Lake Superior is the largest freshwater lake in the world by surface area, covering roughly 31,700 square miles. It holds about three quadrillion gallons of water, which is half of the entire volume of the Great Lakes system. The lake is so deep that its lowest point reaches about 1,333 feet below the surface, and its average depth is around 483 feet.

The lake formed through a different process than most large lakes. About 1.1 billion years ago, North America began to split apart along a tectonic rift in what is now the lake’s basin. The continent did not fully separate, but the failed rift left a deep scar filled with dense volcanic rock. Later glaciers deepened the basin further, but the underlying structure dates to that ancient geological event.

Water moves through Lake Superior very slowly. The average drop of water entering the lake today will take about 191 years to flow out through the St. Marys River into Lake Huron. That means water entering now will not leave until roughly the year 2216.

The lake is consistently cold. Its average surface temperature is about 40 degrees Fahrenheit year-round, and the deep water remains near freezing at all times. More than 200 rivers feed the lake from Minnesota, Wisconsin, Michigan, and Ontario, draining an area of about 49,000 square miles.

Because of the cold temperatures, the lake preserves what sinks into it. Bodies of people who drown in Lake Superior typically do not surface, as the cold water prevents the bacterial activity that normally causes decomposition gases to build up. Similarly, wooden shipwrecks on the lakebed remain in unusually good condition, some for more than a century, because the organisms that break down wood in warmer water cannot survive there.

The lake has a reputation for dangerous conditions. It is estimated that between 350 and 550 ships have sunk in Lake Superior since European navigation began. A stretch of water between Munising and Whitefish Point in Michigan is known as the Graveyard of the Great Lakes. The most famous wreck is the SS Edmund Fitzgerald, a 730-foot iron ore carrier that sank on November 10, 1975, during a storm with winds over 90 miles per hour. All 29 crew members died. No distress signal was sent, and the ship disappeared from radar. The wreck lies in two pieces at a depth of about 530 feet and has never been recovered. Each year on November 10, a bell is rung 29 times at the Mariners’ Church in Detroit in memory of the crew.

During autumn storms, the lake can produce waves regularly exceeding 30 feet. The lake also generates a phenomenon known as the “Three Sisters,” a sequence of three large rogue waves that can strike a ship in rapid succession before the deck drains from the first wave. Marine engineers have suggested this pattern may have contributed to the loss of the Edmund Fitzgerald.

The lake creates its own weather patterns. In late autumn and winter, cold air moving south from Canada passes over the relatively warmer lake, picks up moisture, and releases it as heavy snow on the opposite shore. Michigan’s Keweenaw Peninsula receives up to 300 inches of snow per year, largely from this lake-effect process. The lake also moderates summer temperatures along its shores, keeping coastal communities noticeably cooler than inland areas nearby.

Sudden changes in atmospheric pressure can cause the entire lake to oscillate in a phenomenon called a seiche. This can produce significant water level changes on one shore while the opposite shore experiences flooding, sometimes with little warning.

The land around the lake is still rising slowly. The region was covered by thick continental ice sheets thousands of years ago, and the weight of that ice pushed the earth’s crust downward. Since the ice melted about 10,000 years ago, the crust has been slowly rebounding. The northeastern shore is currently rising at a measurable rate, gradually shifting the lake’s basin.

The lake is unusually clear. It is classified as oligotrophic, meaning it has low nutrient levels, which limits algae growth. Average underwater visibility is about 30 feet, and in some northern areas, visibility can reach up to 100 feet.

Before European settlers named it, the Ojibwa people called the lake “Gitchigami,” meaning great sea or huge water. French explorers in the seventeenth century called it “Lac Supérieur,” meaning upper lake, because it sat above the other Great Lakes on maps. The name was later anglicized to Superior.

Lake Superior sits about 21 feet higher in elevation than Lake Huron. Between them, the St. Marys River runs through rapids that ships cannot navigate. The Soo Locks, built in 1855, were constructed to lift and lower vessels between the two lakes. They remain among the busiest commercial locks in the world and are essential for moving iron ore from mines in Minnesota and Michigan’s Upper Peninsula to steel mills farther east. A significant portion of the steel used in American industry has passed through Lake Superior at some point in its supply chain.

The lake’s shoreline, including its islands, measures about 2,726 miles. That is roughly the distance from Miami to Seattle.

Isle Royale, an island in the lake, is home to one of the longest-running ecological studies in biology. Since 1958, researchers have tracked the predator-prey relationship between isolated populations of wolves and moose on the island, which has no roads or permanent residents. The study has shaped scientific understanding of how ecosystems function in isolation.

Lake Superior agates, gemstones with red banding formed by volcanic activity about 1.1 billion years ago, are found along the lake’s shores. They formed when silica-rich groundwater seeped through gas pockets in cooling basalt, depositing concentric rings of iron oxide. They are found almost exclusively along Lake Superior’s shores and can be collected on public beaches.

A rock formation on the Sibley Peninsula in Ontario resembles a giant person lying on his back, with folded arms and a face turned toward the sky. Known as the Sleeping Giant, it is sacred in Ojibwa tradition. One legend holds that it is a spirit named Nanabijou who was turned to stone.

Ancient red ochre pictographs are painted on a cliff face in Lake Superior Provincial Park along the lake’s eastern shore. The images, which include boats, animals, spiritual figures, and cosmic symbols, were made by Ojibwa people hundreds of years ago. They can only be reached by boat and only when the water is calm.

Because of its depth and continuous wave action, Lake Superior rarely freezes over completely. The last time it came close to full ice coverage was during the polar vortex of 2014. When the lake does freeze partially, the surface becomes silent and still, with ice that shifts and cracks as temperatures change.

The lake’s slow water turnover means that water molecules that entered it in the 1830s, during the early years of American westward expansion, are still there. They have remained in the lake through every storm and shipwreck for nearly two centuries. Water entering today will not leave until the next century.