From the road, Harrington Sound can look almost still — a sheltered expanse tucked inside the island, ringed with homes, gardens, and pale limestone cliffs.
The basin’s origins go back more than 100 million years, to when volcanic activity pushed a mountain up from the Atlantic in two separate phases. The first eruption built a seamount; the second, about 33 million years later, added to its height. Over the ages, that peak eroded, sank, and vanished beneath the waves. What’s left is buried far below, capped with thick layers of marine limestone laid down during warmer, wetter climates when sea levels and weather patterns were very different from today.
Around Harrington Sound, this limestone belongs to the Walsingham Formation — the oldest and hardest rock in Bermuda, more than 700,000 years old. It was shaped when sea levels were low enough for rainwater to seep deep into the rock, slowly dissolving it and leaving behind sinkholes, shafts, and tunnels. This kind of erosion, known as karstification, still shapes the island’s water systems today, affecting where fresh water collects, how it flows, and how seawater moves underground.
The Sound itself is a wide, shallow basin, roughly two miles long and a mile and a half across, with an average depth of just under 15 metres. At Devil’s Hole, the floor drops to almost twice that. The shoreline changes from soft, sandy slopes to sudden rocky ledges. In the shallows, seagrass meadows grow thick, while deeper edges give way to rubble and coral.
On the inner flats, beds of turtlegrass anchor the seabed and trap fine sediment, where they serve as a nursery for young fish and invertebrates. A little further out, coral patches appear, including orange cup coral (Oculina), which does well in lower light. Sponges, small sea fans, and filter feeders like tunicates and hydroids thrive here. Beyond 19 metres, light and oxygen levels drop, and only a handful of species are adapted to survive in the low-energy conditions.
Though the inlet at Flatts is only about 20 metres wide, the Sound is anything but isolated. Seawater moves in and out through extensive submerged cave systems. These unseen channels, cut through the limestone, carry the tide in slow pulses.
To the east, the Sound links with the Walsingham and Palm cave systems near Castle Harbour. To the north, Green Bay Cave runs beneath the peninsula that separates the Sound from the North Shore — the longest known cave in Bermuda. To the south, Devil’s Hole connects to the ocean through fissures beneath the surface. The tides inside the Sound rise and fall with less force than the open sea and several hours behind it.
Green Bay Cave is among the clearest underwater environments in Bermuda, with visibility so high that the water’s surface can be difficult to distinguish from below. The ceilings are lined with large stalactites, formed during glacial periods tens of thousands of years ago when sea levels were lower and the caves were dry. In some chambers, the floor drops abruptly into unlit spaces, part of an extensive network of submerged passages that make up much of Bermuda’s hidden geology.
Life in the Sound has adapted to these conditions. Juvenile wrasses, grunts, and parrotfish weave through the grass beds; crabs, sea stars, and conchs forage along the bottom. On the slopes, sponges grow in irregular forms shaped by the slow currents, and tunicates cling to shaded overhangs where plankton drifts past. Inside the caves live species found nowhere else, including the blind cave shrimp Mictocaris halope.
The Sound’s human story is just as layered. Early maps mark it as “Little Sound,” but by the mid-1600s it was called Harrington Sound, named for Lucy Harington, Countess of Bedford, a Jacobean courtier and an early investor in the Somers Isles Company. Historical records suggest she was the first shareholder granted land in this part of the island, and her name — later settled into the now-familiar double “r” — stayed.
For centuries, its calm waters served as a harbour and a source of food. The rocky bottom supported fisheries for lobster, conch, and shellfish, but one of the most important catches was the Turkey-wing mussel (Arca zebra). In the past, these were collected by free divers using nothing more than a basket. In the 20th century, dredges pulled behind small boats made the work faster, though today the fishery is smaller and managed, with hand-diving preferred to limit damage to the seabed.
Flatts Inlet, the Sound’s visible link to the ocean, has long been a natural crossing point. Flatts Bridge connects the North Shore with Flatts Village and the winding roads that circle the basin. The Bermuda Aquarium, Museum and Zoo sits nearby, its exhibits offering a view of the tide moving through the inlet — a small example of the slow, steady exchange that defines the Sound.
Stories cling to the water too. At Devil’s Hole, changes in wind, tide, and air pressure can produce low, rumbling noises. Earlier generations credited the Devil. From the early 1800s until 2009, the site operated as a tourist attraction and natural aquarium, with wild fish and turtles swimming in and out through hidden channels. It is now closed to the public, though the legend remains.
Today, the Sound is part of daily life as much as it is a subject for research. Kayakers roam along its edge, snorkellers drift over the grass beds, school groups come to learn about the island’s geology and marine life. For scientists, it is a natural laboratory, a rare example of a semi-enclosed marine system. For everyone else, it is simply another part of our island home.
