Mbeli Bai is a swamp clearing of thirteen hectares in the rainforest of the Nouabalé-Ndoki national park, in the Sangha department of northern Republic of the Congo, where western lowland gorillas walk out of the trees and wade chest-deep for water plants, and where a gorilla was first seen using a stick as a tool, watched from a plank platform nine metres up at the clearing's edge. The bai is sedge and black water walled by forest on every side, and it has been watched every day since 1995 by the longest-running study of wild gorillas in the lowland forest; the researchers wear no collars on the animals and carry no tags, recognising each of the several hundred gorillas that use the clearing by the wrinkles on the nose, so that families are known by name across generations. The gorillas come in family groups behind a silverback, walking upright into the swamp on two legs, sinking to the chest in the black water and hauling up fistfuls of the herbs, and it was here in 2005 that a female was photographed testing the depth of a pool with a stick before wading in, the first tool use recorded in wild gorillas.
Forest elephants wade into the pools they have excavated themselves, drink, spray and sink until little more than a trunk shows above the surface, and sitatunga, forest buffalo and bongo come out at the edges. The platform is climbed before dawn and the day is spent on it, since the bai fills and empties at its own pace, and the walk in from the camp crosses a forest of elephant trails; the park is reached by boat and road from Ouesso, with the habituated gorillas of Mondika a day further on. August is named by the park as the best month, when the fruit peak draws the families to the clearing most reliably, even in the long rains. Mbeli Bai is a swamp the gorillas walk into, and the platform has watched them for thirty years.