On Santa Cruz Island, you can walk through lava tunnels.
On our last hike to Volcan Chico (Isabela Island) we saw smaller versions of these tunnels: lava tubes.
Our guide explained that while the lava flow begins to cool and harden on the outside, the lava remains hot and liquid inside. Eventually the hardened lava forms a solid rock tube that keeps the lava insulated from the cooler outside air.
There are remnants of these tubes in the lava fields of Volcan Chico. Much of the tubes remain intact – and look like a giant fossilized snake.