Board Thread:Help Desk/@comment-24000921-20131009020320/@comment-1250011-20131010064320

What you've found is the Jungle Temple. It is a Hard Mode alternate dungeon that contains Lihzahrds, Flying Snakes and the Golem boss. To get into it, you must enter Hard Mode by killing the Wall of Flesh and then defeating Plantera, who drops the Temple Key.