Caves

Most sit in Spain, and most of those along the Mediterranean coast in Valencia and Murcia, with a cluster of prehistoric burial caves on Menorca. Iceland supplies the rest: lava tubes under old lava fields, and one ice cave under the Katla glacier. Some are show caves with lit walkways and a ticket desk — Nerja stretches almost 5 kilometres, Saint Joseph's Caves hold Europe's longest navigable underground river. Others are bare rock you enter with a headtorch. Check which you are getting before setting out: some Icelandic tubes are guided-tour only, others free, unlit and unguided.