To Sua is a pool of clear green water sunk thirty metres into a lava field on the south coast of Upolu, Samoa's main island, and nothing about it is spring-fed. The water in it is the Pacific, arriving through the rock. It also began as a dome rather than a hole. As lava drained away through a tunnel network running seven or eight kilometres along this coast, molten rock backed up underneath and lifted the cooling crust into a blister as much as sixty metres high. When that roof finally gave way it fell in, leaving a round shaft about thirty-five metres across. A smaller dry twin sits nearby, To Le Sua, the deep hole without water. The ground on top is flat garden, so there is no warning until you are standing at the lip with ferns and vines spilling over every ledge below you.
A tall wooden ladder bolted to the rim is the only way down, and you back down it rung by rung to a small floating platform at the water. Out in the middle you can lie back and let the pool hold you still, the walls rising sheer for thirty metres to a single ring of daylight. On the shaded side a low rock opening marks where the hidden channels run out to the sea, and swimming toward it you feel a gentle surge, or meet a small reef fish. That is the proof of what the pool is. Because the ocean seeps in slowly through old lava rather than pouring over an open shore, the surface stays glassy even while it rises and falls with the tide. On a big swell you can feel that pulse reach all the way in. Across the road the same tunnel system does the opposite. The lava field there is bare black rock where the Pacific breaks, and the surf forces up through openings in it and falls back into shallow pools calm enough to sit in. The same water reaches the trench behind you without a sound.