The Jungle, not quite a tropical rainforest, covers over 50% of the island's surface.
Mostly grown-in to the point of it being extensively hard to travel through, there are a couple of walking paths that go at most a mile into the edges of the jungle. The trees are tall, with vines draped across their branches, and roots that portrude well above the surface of the ground. Undergrowth is very heavy--bushes, ferns, herbs, things that grow on trees, other assorted things that don't grow on trees, things that are poisonous, things that aren't poisonous, the general undergrowth you might expect of a place like this. A good many freshwater streams come down in rills from the northern parts of the island--the only freshwater on the island. Some of it pools into lakes, some of it runs all the way out to the ocean. The largest area that does is dammed for hydroelectricity.
The land itself slopes harshly downward as it moves toward the volcano, and then slopes back up again in the northern areas, creating almost a wall around the volcano's base, carved out from the lava flow.
Densely populated by dual Dark-types, the most notable are the sheer amount of Carvanha and Corphish in the waters. Nuzleaf, Murkrow, Stunky, and Purrloin join an abundance of bug-types in the undergrowth and up in the trees, as well as the occasional Houndour closer to the volcano. While the waters are most definitely deemed not safe (except in dry season, when most of the waters dry up and the Carvanha are harvested for Fish Frying), the other Pokemon tend to be indifferent, but will respond negatively if provoked.
The furthest exploration ever done of the jungles was by the young Navy men who had discovered it in the first place--they took accurate maps of what they found, recorded all of the life that they saw (including some of the poisonous ones), and had it published in a document when they returned to the mainland, which is now included in every traveler's guide to Poyi as a downloadable .pdf from its website. Although efforts by researchers have been made before, they have not gotten as far as they would like to, due to the natives keeping such a close watch over their territory. Any brave explorer daring enough to take that trek, though, is welcome to, and report their results back.