Mbeli Bai is a swamp clearing of thirteen hectares in the rainforest of northern Congo, and what you do there is sit still and watch. You climb a plank platform nine metres up and look out over sedge and black water, walled by forest on every side. Nothing here is habituated to people, so you watch from up there or not at all. Sound carries, and a branch cracking at the tree line usually tells you something large is about to step out of it. The clearing stays open because no tree can root in ground waterlogged all year, and the animals that come to feed keep churning it. Gorilla families walk out of the trees on two legs, sink to the chest in black water and haul up fistfuls of the aquatic herbs. A silverback standing upright in a pool, arms out for balance, is the sight this place is known for.
Forest elephants wade into pools they dug themselves, drink, spray and sink until little more than a trunk shows. The digging is the point. Those pits reach dissolved salts, so an animal will stand in a muddy hole and ignore the clear water at its feet. They keep to twos and threes in the trees, so this is one of the few places a calf meets elephants it is not related to. Their tusks grow straight and point downward, no two alike; with the tears in an ear and the scars on a flank, every bull out there is a known individual. The gorillas are known the same way, by the pattern of wrinkles above the nostrils, with no collar or tag on them. 514 have been named since 1995, and infants born in front of the platform have come back with families of their own. One October morning in 2004 a female called Leah stepped into one of those pits, went back for a branch and used it to sound the depth ahead. No wild gorilla had ever been recorded using a tool, and it was seen by people who had done nothing all morning but sit there.