Mainland is the large area outside of the starter islands where most of the game's action is supposed to take place. A bridge can be used to get to the mainland. It is also possible to swim from the Iridium Ocean to a ladder supporting the mainland, and in some uncommon cases, you can jump to the Mainland.
Destroying all of the supports on the Mainland will cause the Mainland to fall into the Iridium Ocean below and destroy it (NOTE: while destroying said mainland you may find blocks that seem unbreakable but they're not, they're just very strong due to their VERY large size, and it's probably a smart idea to not try to destroy it for it will take forever). Please note that once the Mainland begins to fall, you may experience extreme lag. Some users may experience game crashes.