Boin Archipelago

The Boin Archipelago is a series of flower-shaped islands in the Grand Line. Within them are various gigantic bugs and vicious man-eating plants. Usopp was sent to one of these islands by Bartholomew Kuma during the events of Sabaody Archipelago, and spent two years on the island honing his skills. It is said that anyone who enters this island has no chance of leaving it again, though both Usopp and Heracles managed the feat.

Geography
The island Usopp was sent to is home to the swindling forest of Greenstone (グリンストン). Also, as of recent chapters, it is known to have a forest made of food, called the Forest of Gluttony. This forest not only includes ordinary wild food such as Plum Trees and Corn Stalks but also a river of already prepared ramen. In the anime, this forest resembles a food-themed theme park more than anything else. Although it's mentioned there are thieves in Greenstone, the only known resident so far is Heracles.

The islands are actually giant carnivorous plants scientifically known as "Stomach Baron", that eat overweight animals that are in the Greenstone forest. A group of giant beetles guard the edges.

Anime and Manga Differences
The Boin Archipelago is first revealed to be a group of giant, seaborne carnivorous plants in anime Episode 455. This was later revealed to be canon in manga Chapter 591. The only difference is that the anime originally depicted the island's center as having teeth, while the manga shows the island with no teeth whatsoever. This was corrected in Episode 507 where the island had no teeth as in the manga.

Trivia

 * It uses several methods of trapping that real life plants use. The method of luring via tasteful fruit is yet another reference to these carnivorous plants as many use sweet scents or sticky mucilage or glue to lure insects to them. Its leaves resemble plants like the pinguicula, however its method of entrapment more closely resembles the Venus Flytrap.
 * The idea for a carniverous island that lures and fatens prey may have been taken from the Captain Bluebear series of stories, where the hero spends his 4th life Gourmet Island.