The Sargasso Sea, located in the North Atlantic Ocean, is the only sea on Earth with no coastline. Instead of being bordered by land like every other sea, its boundaries are formed entirely by four rotating ocean currents that trap a distinct body of water in the open ocean. It sits roughly between the Caribbean and the middle of the Atlantic, covering an area that shifts with the currents but spans roughly 2 million square miles.
How Ocean Currents Create a Sea
The Sargasso Sea exists because of the North Atlantic subtropical gyre, a massive system of clockwise-flowing currents. The Gulf Stream forms its western edge. The North Atlantic Current defines the northern boundary, the Canary Current runs along the east, and the North Atlantic Equatorial Current closes the loop to the south. Together, these currents act like invisible walls, circling the water and keeping it relatively contained.
Water trapped at the core of this gyre can remain there for up to 50 years before escaping, according to estimates from the Sargasso Sea Commission. This long residence time gives the sea its distinctive character: it’s calmer, warmer, saltier, and far clearer than the surrounding Atlantic. The deep blue color is a result of very low nutrient levels, which means less plankton to cloud the water. Subtropical gyres like this one cover roughly 65% of the ocean’s surface, but the Sargasso Sea is the only one dense enough with life and distinct enough in character to earn its own name.
The Golden Seaweed That Gives It a Name
What makes the Sargasso Sea visually and ecologically unique is its thick mats of free-floating golden-brown seaweed called Sargassum. Unlike most seaweed, which anchors to the seafloor, Sargassum in the Sargasso Sea is holopelagic, meaning it lives its entire life cycle drifting at the surface. The circling currents concentrate it into dense rafts that can stretch for miles.
These floating mats function as an ecosystem unto themselves. Multiple species found nowhere else on Earth have adapted to live among the seaweed, including small crabs, shrimp, flatworms, and fish that have evolved camouflage matching the Sargassum’s color and texture. The surface-dwelling community, known as the neuston, also includes floating barnacles, snails, and jellyfish-like organisms. For open ocean, this level of biodiversity in a single habitat is remarkable.
A Critical Breeding Ground for Eels
The Sargasso Sea plays a role in one of nature’s most extraordinary migrations. Both American and European eels travel thousands of miles to spawn in this patch of open ocean, and nowhere else. The Danish biologist Johannes Schmidt first proposed the Sargasso Sea as the eels’ breeding ground nearly a century ago, based on surveys that found the smallest, newly hatched larvae concentrated in a band between roughly 24°N and 31°N latitude, from 50°W to 70°W.
For decades, no one could confirm this directly because no adult eel had ever been tracked all the way there. That changed in 2022, when researchers attached satellite tags to 26 European eels released from the Azores and tracked them for up to 366 days. Some of those eels reached the Sargasso Sea, providing the first direct evidence of adult European eels completing the journey to their presumed spawning grounds. European eels travel up to 5,000 kilometers to get there, making it the longest and most complex ocean migration of any eel species. The European eel is now critically endangered, having suffered a 95% decline in young eels arriving at European coasts since the 1980s.
Columbus and the “Sea of Weeds”
Christopher Columbus provided the first written account of the Sargasso Sea in 1492, during his initial voyage across the Atlantic. His ship, the Santa Maria, along with its two companion vessels, became becalmed in the windless center of the gyre for three days. His sailors, who associated floating seaweed with shallow coastal waters, panicked. They feared they would run aground on hidden reefs, become tangled in the weed, or be dragged beneath the surface. Columbus himself noted that the persistent calms could potentially prevent the fleet from returning to Spain. These early encounters gave the Sargasso Sea a fearsome reputation among European sailors that persisted for centuries.
Plastic Pollution in the Gyre
The same circular currents that concentrate Sargassum also trap floating debris. Scientists first reported high concentrations of plastic in the Sargasso Sea in 1972, when researchers from the Woods Hole Oceanographic Institution measured up to 12,000 plastic particles per square kilometer. The problem has only grown since. More recent surveys spanning 22 years of data found average plastic densities exceeding 20,000 pieces per square kilometer in the Sargasso Sea, comparable to the more widely publicized Great Pacific Garbage Patch. The highest single measurement recorded was 580,000 pieces per square kilometer east of the Bahamas. Overall, about 83% of all plastic debris collected across the North Atlantic was concentrated within the subtropical gyre.
Who Governs a Sea With No Country?
Because the Sargasso Sea has no coastline, no single nation has jurisdiction over most of it. Bermuda sits within its boundaries and controls the surrounding exclusive economic zone, but the vast majority of the sea falls in international waters. This creates a governance gap: fishing, shipping, and pollution are regulated by a patchwork of international bodies rather than any unified authority.
In 2014, several governments signed the Hamilton Declaration, a non-binding political agreement that established the Sargasso Sea Commission. The Commission has no direct management power. It serves a stewardship role, monitoring the sea’s health, productivity, and resilience while encouraging voluntary collaboration among nations. Actual regulatory authority still rests with existing organizations like the International Maritime Organization for shipping and the International Commission for the Conservation of Atlantic Tunas for fisheries. Protecting a sea that belongs to no one remains one of ocean conservation’s more difficult challenges.