The bay's origin story begins 35.5 million years ago, when a bolide struck the shallow coastal sea at what is now the bay's mouth, forming the Chesapeake Bay impact crater. That buried depression helped predetermine the course of rivers for millions of years. But the bay as we know it is far younger - roughly 10,000 years old, formed when rising sea levels at the end of the last ice age drowned the Susquehanna River valley. The Chesapeake is technically a ria, a drowned river valley, not a fjord - the Laurentide Ice Sheet never pushed this far south. The result is a body of water stretching from the Susquehanna's headwaters to the Atlantic, spanned by the Chesapeake Bay Bridge in Maryland and the Chesapeake Bay Bridge-Tunnel in Virginia, which crosses the mouth on man-made islands and tunnels that allow ships to pass unimpeded.

The Chesapeake built a culture around its bounty. Blue crabs, oysters, clams, and rockfish - the regional name for striped bass - sustained generations of watermen who worked from skipjacks and other shallow-draft boats designed for the bay's modest depths. In the mid-20th century, 9,000 full-time watermen harvested these waters. The oyster trade rivaled crabbing, and the competition over oyster beds was so fierce that the late 19th and early 20th centuries saw actual "Oyster Wars" between Maryland and Virginia harvesters. Rockfish nearly went extinct before a legislative moratorium allowed populations to recover; they can now be fished only in strictly controlled quantities. The name "Chesapeake" itself is an Algonquian word - not meaning "great shellfish bay" as widely believed, but likely something closer to "great water" or simply referencing a village at the bay's mouth. The Chesepian people who inhabited the area around present-day Norfolk gave their name to waters they fished for millennia before any European arrived.

The bay has witnessed pivotal moments in American history. In 1781, the French fleet defeated the Royal Navy at the Battle of the Chesapeake - the decisive naval engagement of the Revolutionary War - enabling George Washington and the Comte de Rochambeau to trap Cornwallis at Yorktown and end the conflict. During the War of 1812, the British treated the bay as their personal lake, raiding towns from their base on Tangier Island. Admiral George Cockburn's forces eventually landed on the Patuxent, marched overland, routed the U.S. Army at Bladensburg, and burned the Capitol in August 1814. Underwater archaeologists have since cataloged over 1,800 shipwrecks on the bay's bottom, from precolonial canoes to Revolution-era warships scuttled by Cornwallis in the York River. In 1974, scallop fishermen even dredged up the skull of a 22,000-year-old mastodon.

In the 1970s, scientists identified one of the planet's first marine dead zones in the Chesapeake - waters so oxygen-depleted they killed everything in them. Dead zones now destroy an estimated 75,000 tons of bottom-dwelling clams and worms annually, collapsing the food chain beneath the bay's iconic blue crabs. Nutrient pollution from farm runoff, urban development, and sewage overflows feeds massive algal blooms that block sunlight and suffocate the water when they decay. Eelgrass beds in the southern bay have shrunk by more than half since the early 1970s. Oyster reefs that once naturally filtered the entire bay's volume in days have been reduced to a fraction of their former extent. Water that was once clear enough to see the bottom is now so turbid that a wader loses sight of his feet before the water reaches his knees. The sediment record shows nutrient loading increasing since the 17th century - this is not a new problem, just one that has reached critical mass.

Restoration has been underway since the 1980s, when EPA declared the bay an "ecosystem in decline." The Chesapeake Bay Agreement of 1983 united Maryland, Virginia, Pennsylvania, the District of Columbia, and EPA in a partnership that continues today through the Chesapeake Bay Program. Experimental oyster reefs created in 2004 by the Virginia Institute of Marine Science now house 180 million native oysters - impressive, but a fraction of the billions that once thrived. Sewage treatment upgrades across the watershed have reduced nitrogen and phosphorus pollution despite growing population and wastewater volume. Dam removals have reopened tributaries for migratory fish. In 2023, the University of Maryland Center for Environmental Science gave the bay its highest health rating since 2002 - a C-plus, 55 out of 100. It is the kind of progress measured not in victories but in slightly slower decline, a fight waged across six states and decades against the accumulated consequences of 400 years of human use.