Just Hangin' (くさいぞ！ げすいどう lit. The Sewer is Stinky! in Japanese) is the eighth level and the third stage of the second warp room in Crash Bandicoot: The Huge Adventure for the Game Boy Advance. It takes place in a sewer.
The green gem is needed to get both clear gems. This is because the green gem path contains a second clear gem in it, as well as 47 of the level's crates, and therefore, it's also necessary to obtain the level's box gem.
To find the green gem platform, Crash must jump over at the end of the level's exit and then double jump over to the platform. Since the green gem is in Sunken City, Crash will have the double jump if he has this gem.