Volcanoes are the sources of valuable gases in the Underwater world. The blue volcanoes provide oxygen, it can be consumed by your base and converted into money. Also the squirrel wheels consume oxygen to rotate.
Volcano reacts to taps: in the beginning every 3.9 taps produces one bubble. The more bubbles are produced, the more taps are needed for the next one (+0.001 tap per bubble created)
Volcano has 4 slots for a Hammer (Underwater).