The São Vicente Caves were formed 890,000 years ago by a volcanic eruption in Paul da Serra. The outer part of the lava that was exposed to low temperatures hardened rapidly while the inner part remained liquid with a lot of gases, forming a series of lava tubes, which are among the first Caves of volcanic origin, which are open to the public in Portugal, can be visited by following an underground path that extends for more than 1,000 metres.