The name covers six distinct bodies of water strung together along the coast: Tam Giang, Thanh Lam, Sam Chuon, Ha Trung, Thuy Tu, and Cau Hai. Each has its own character, its own depth, its own relationship to the sea. The whole system connects to the South China Sea through two inlets: Thuan An, near the center, and Tu Hien, at the southern end. These openings govern everything — the salinity of the water, the tidal rhythms, the movement of fish. The lagoon is generally shallow, between one and three meters deep across most of its expanse, though it deepens dramatically near the Thuan An inlet, where the channel reaches 11 meters. At that inlet, the meeting of lagoon and sea is turbulent, the water churning between tidal forces pulling in opposite directions.

The Perfume River — Sông Hương — carries the weight of Huế's imperial history for 30 kilometers before it releases into this lagoon. That arrival is quiet. The river, which has been moving almost imperceptibly slowly by the time it reaches the coast, simply widens into the lagoon's northern reaches. The distinction between river and lagoon blurs. Other smaller rivers and streams feed the system from the west, bringing freshwater down from the Annamite mountains, mixing with the brackish tidal flows from the sea. This constant exchange between fresh and salt water creates conditions that support an enormous variety of aquatic life — the lagoon's productivity is one reason so many people have historically chosen to live on its margins and its floating villages.

For the communities that have lived alongside the Tam Giang–Cau Hai lagoon for centuries, the water has never been a backdrop — it has been the floor of daily life. Floating villages cluster on the lagoon's surface, clusters of homes and boats so integrated that the distinction between dwelling and vessel sometimes disappears. Fishermen work the shallows with traditional cast nets and long-lines, targeting shrimp, crab, and the various fish species that breed in the brackish shallows. The lagoon's enclosing sandbar — the thin strip of land between the water and the open sea — supports a string of coastal villages whose residents have always lived between two bodies of water, neither fully of the sea nor of the land. The system's productivity has attracted aquaculture operations in recent decades, with fish and shrimp ponds carved into the margins, though the relationship between traditional fishing and modern aquaculture has not always been easy.

Numbers help here: 22,000 hectares of water surface is roughly the size of a medium-sized city's metropolitan footprint. Nearly 70 kilometers in length means a traveler driving Vietnam's coastal Highway 1 passes alongside this lagoon for the better part of an hour without reaching its end. The visual effect from the road is striking — the lagoon opens to the west, flat and still in the morning hours, while the narrow sandbar blocks any view of the sea to the east. Villages appear along the shore at irregular intervals, boats anchored in the shallows, the occasional fishing platform extending on wooden stilts over the water. The lagoon is not a wilderness; it is a working landscape, shaped by centuries of human use. But its scale — the wideness of the water, the lowness of the horizon — gives it a quality of openness that the mountainous interior of central Vietnam does not.