Mbeli Bai is where a gorilla first showed the world it could use a tool. On 9 October 2004, researchers on the platform watched an adult female named Leah walk into a pool an elephant had dug, break off a branch about a metre long, and prod the water ahead of her to test how deep it ran — then lean on it as a walking stick to wade across. Nothing like it had ever been recorded in a wild gorilla. Leah was crossing a swamp clearing of 12.9 hectares, an opening in the forest canopy of Nouabalé-Ndoki National Park. The clearing lies in a block of northern Congo rainforest that no timber company ever cut, and UNESCO listed that forest in 2012 as part of the Sangha Trinational. Clearings like this stay open because the ground is waterlogged and loaded with minerals the surrounding forest cannot supply. Gorillas wade in for protein-rich aquatic herbs; forest elephants dig pits in the mud to reach dissolved salts.
A study team has watched the clearing from a 9-metre platform at its edge since February 1995, for around nine hours a day and more than 77,000 hours in total. They now know 514 gorillas by sight and have followed the same families across generations. Over those years the population they count has climbed from 101 to 226. Visitors watch the same way the researchers do: up the ladder before the mist lifts, silent, the far tree line a few hundred metres off. Elephants come out of it one at a time and stand belly-deep, spraying. Nothing here is habituated, and that is the point — these are wild animals having an ordinary morning. Reaching them is still an expedition of charter flights, river boat and forest trail, and the park takes only a handful of visitors on set dates.