Huge underground lava tubes perforate Santa Cruz, and in places extend for several kilometres enclosed by high jagged walls that disappear into the gloom. The tubes were formed when cooler outer parts of the lava flow hardened into thick rock walls, providing insulation to keep a flow going inside. Eventually, the flow subsided, leaving long empty tunnels easily big enough to walk down. As they’re on private land, visitors don’t need an official guide to explore these volcanic curiosities, though tours can be arranged in Puerto Ayora. The floors can be slippery and rubble-strewn, so a flashlight and sturdy shoes are a good idea.