Sunken City (かいてい たんさ lit. Exploration Undersea in Japanese) is the thirteenth stage and is the third level in the third warp room in Crash Bandicoot: The Huge Adventure. It is also the third underwater-themed level in the game.
As the name suggests, it takes place in a sunken city as shown by the fallen pillars. The green gem is located here. How to get it was inspired by Deep Trouble from Crash Bandicoot: Warped, except for the fact that the game left out everything that made the gem hard to get.
Getting the Green Gem
There will be a metal ! crate. Activate it and go back to the start of the level. There will be some metal crates, TNT crates and the green gem beneath them. One of the TNT crates is left outside of the metal crates, unlike the others that are between them. Detonate it to unblock the way to the gem.