It occures in a tunnel under tens of meters of vulcanic deposits (mostly Thephra i think) at Herculaneum, Italy. Water seems to come from the surface, however the sea is near. It's level is lower though.
closed as off-topic by Universal_learner, arkaia, Fred, Peter Jansson, Jan Doggen Jul 15 at 7:24
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- "Please review our rock identification guidelines to provide the missing information so that your question is both answerable and useful to new users." – Universal_learner, arkaia, Fred, Peter Jansson, Jan Doggen