Wave Echo Cave is hidden among the Sword Mountains near Phandalin.
The cave is rather appropriately named for the sounds of waves that echo through it. The cave is surprisingly cold. It has a portion nicknamed "jack-off tunnel" for the sound that the jelly monsters make when they are following the Tres Horny Boys.
Through jerk-off tunnel, one of the caverns has a deep spring well into which a drop of water falls from a stalactite, causing the unusually big waves after which the cave is named. Around the spring are fungi that release spores in response to noise. Above the spring is a wooden elevator.
Flora and fauna of the cave include said jack-off jellies, fungi, and several dwarf skeletons.
Also Magic Brian.