Lac Salé fills a crater at the northern tip of Grande Comore, in the Comoros, and it rises and falls with the tide although nothing connects it to the sea. No stream feeds it. Seawater works through the porous lava between the lake and the shore, and sitting in the sun it has grown saltier than the ocean outside. The colour will not hold still: green at first light, hard blue once the sun is over the rim, grey the moment a cloud crosses. The crater is one of dozens. The northern third of the island is a field of small cones, most built by a single short eruption and never used again.
A rough path runs right around the lip, unfenced, the salt lake on one side and the open Indian Ocean on the other. Ten minutes below are the beaches at Maloudja and Mitsamiouli, white sand under coconut palms all bent one way by the trade wind. The fringing reef starts a short swim out, shallow enough that you can pick out the coral heads from the sand before you go in. Under the rim the lava breaks into coves where people from the village cool off, the black rock holding the day's heat. From November to April the monsoon throws walls of rain across the coast, and from up here you watch one cross the water long before it arrives. Between July and October humpbacks come up from Antarctic water to calve in the channel, and the boats cut their engines and wait. A mother lies almost still while her calf rolls and slaps beside her, and the blow reaches you before you find it. Comorian tradition says a village stood in this hollow until a barefoot stranger came asking for water, and every household turned him away but one old woman's; that night the water rose over the rest. Halfway round the rim the ridge narrows, and it is plain how little rock is keeping the two waters apart.