Bioko is roughly oval-shaped, about 70 kilometres long and 30 kilometres wide - a bit smaller than the island of Jamaica. Because it formed from volcanic lava, the soil is very rich, and almost everything grows there: giant ferns, huge trees, and flowers in every colour. The forest is so thick that some paths are dark even in the middle of the day.
Three old volcanic calderas - which are like giant bowls left behind after a volcano stops erupting - sit in the southern part of the island. The biggest one has a lake inside it and you can walk around the rim looking down into the green bowl below. These craters have not been active for a very long time, so they are completely safe to visit.
Because Bioko has been an island for hundreds of thousands of years, animals that live here had time to become slightly different from animals on the mainland. Scientists call these unique versions 'endemic species'. Several kinds of monkey and bushbaby found on Bioko are not found anywhere else on Earth. That makes the island very exciting for wildlife researchers.
The beaches on the southern coast of Bioko are among the most important nesting spots for leatherback turtles on the entire continent of Africa. Every year, turtles swim enormous distances across the Atlantic Ocean to lay their eggs in the sand here, just as their parents and grandparents did before them.