A yawning, kilometre-long chasm, the Cueva de los Verdes is the most spectacular segment of an almost 7km-long lava tube left behind by an eruption 5000 years ago. As the lava ploughed down towards the sea, the top layers cooled and formed a roof, beneath which the liquid magma continued to slither until the eruption exhausted itself. Guided 50-minute tours, in Spanish and English, run every 30 minutes; you'll wander through two chambers, one below the other.
The ceiling is largely covered with what look like mini-stalactites, but in fact no water penetrates the cave: the odd pointy extrusions are where bubbles of air and lava were thrown up onto the ceiling by gases released while the boiling lava flowed, and, as they hit the ceiling, they hardened in the process of dripping back down.
Anyone with severe back or claustrophobia problems might think twice about entering the cave – there are a few tight passages that require you to bend at 90 degrees to get through, although only very briefly. No advance bookings are taken, so you may have to queue for a tour; it's quietest from 3pm onwards.
Concerts are held here between November and February. It's 5km northeast of Arrieta.