Islas Columbretes

Diving the Columbretes is a committing day rather than an outing. Boats leave the Castellón coast early, the crossing takes about three hours each way, and the day is built around two dives with the surface interval spent moored inside the crater harbour. Everything needed goes with you, since there is no shop, no fill station and no second chance if a piece of kit was left on the mainland. Landing on the islands is tightly restricted.

The rock is volcanic and behaves like it. Walls and slopes fall to around 40 m, split by arches and swim-throughs such as the tunnel at La Foradada, and the standout is El Carallot, a bare chimney rising from the seabed with open water on every side. Inside L'Illa Grossa the horseshoe rim gives sheltered walls that work when the outer sites are exposed, and the outlying rock at La Ferrera holds a dense grouper population around its arches.

Two things make the diving distinctive: the water and the fish. With no rivers, no coastal runoff and 50 km of open sea between the archipelago and the mainland, visibility is better than anywhere along the Spanish mainland coast, and the reserve's daily permit cap keeps the sites unpressured. Current is usually mild, so it is depth and the crossing, rather than the conditions on the day, that set the difficulty.

It suits divers willing to trade convenience for a reserve left alone since 1990, and anyone more interested in big grouper, clear water and volcanic topography than in a long list of sites. Open Water divers can dive it once the permit is arranged, though the drop-offs run well below the shallow rims and Advanced leaves the second dive of the day less constrained. Operators run May to October, and summer places go early.