Mount Karthala catches up to 9,000 millimetres of rain a year on its summit, and yet the island below it has no river at all. The volcano's lava is young and full of holes, so the rain sinks straight through, drains underground and comes out at sea, leaving Grande Comore to live off cisterns and wells. The mountain is the island: a shield volcano 2,361 metres high, and one of the most active anywhere, with more than twenty eruptions since the early 1800s. Its summit carries a caldera roughly 3 by 4 kilometres across, walled by layered lava cliffs and floored in grey ash, with the deep Chahalé pit crater sunk into the middle of it. Four eruptions between 2005 and 2007 reworked that floor and dropped ash on the capital, Moroni. Karthala has been quiet since, though an earthquake swarm that began in late 2021 ended fourteen years of calm and put the volcano observatory back on watch.
Walking up means crossing the whole island in a day. Coastal farms give way to thick mountain rainforest, where four birds live that exist on this one slope and nowhere else, then to a belt of giant heather bent flat by the wind, then to bare black rock. Most people set out from M'vouni or Boboni with a guide, climb for about seven hours and sleep in a tent near the rim. Dawn is the moment: before the cloud rebuilds over the summit, you look down sheer cliffs into an ash-grey basin that still leaks steam, and out over the cloud layer to the neighbouring islands. From July to October, humpback whales calve in the deep water below the lava coast.