The Equator Monument marks the point where the equator crosses through the city — one of the few places in the world where a major river city sits directly on the earth's midline. The concrete marker and painted line on the road are modest in themselves, but the concept is not: standing here you are equidistant from both poles, in the heart of the Congo Basin, on the largest river in Africa by discharge.
The monument is on the main road through Mbandaka and easy to find. Vendors typically gather around it selling souvenirs, and it's one of the few places where tourists are expected and catered to (in a very minor way). Locals photograph themselves straddling the equator line, as do foreign visitors. It is a good place to simply stand and think about where you are — at the centre of the continent, in one of the most remote cities accessible by river on earth.