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 lava flows 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, you 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.
One of the easiest to get to is near Bellavista (follow the “los túneles” signs from the village), where you can hire a flashlight ($0.50) to explore a tunnel ($2) several hundred metres long. Another set, known as El Mirador, is within walking distance 2.5km from town ($1 by taxi). There are also the Salasaca tunnels ($3), 5km northwest of Santa Rosa on the farm of Señor Arias; get in contact with him through one of the agencies to arrange a visit.