Gruta dos Ratões

The boat drops you directly over the cave mouth at 15 metres, on the north side of the westernmost Ilhéu das Cabras islet, a short crossing from Terceira's south coast. Swim in and the ceiling rises, the floor climbing to around 12 metres inside a wide volcanic chamber. Sunlight enters through crevices overhead, and through the summer months that light is broken by the silhouettes of eagle rays and round stingrays moving through the water column above you, sometimes a dozen at once. Exit the chamber and the site opens onto a mixed boulder-and-sand seabed running out to 24 metres, patrolled by schools of almaco jack, yellowmouth barracuda and bluefish.

Only a few places in the whole Azores archipelago have a documented mass reproductive aggregation of eagle rays, and the cave takes its name from them. "Ratões" is the Azorean word for the rays, not for rats, despite what the slug suggests. It also breaks the usual rule for a site called a gruta. The chamber is large enough to swim or motor a boat straight into, so no cave or penetration certification applies, and it is rated fine for snorkelling too. Few sites anywhere pair a genuine rare-species draw with that low a technical barrier.

Grade the dive to where you want to go: the interior at 12-15 metres sits within Open Water limits, but the exterior section beyond the cave mouth reaches 24 metres, so continue that far only with at least Advanced Open Water. Current is generally weak. Some divers carry a light to pick out the darker recesses of the chamber. The islet sits inside a Natura 2000 Special Protection Zone, an Azores UNESCO Global Geopark geosite and an Important Bird and Biodiversity Area, a different regime from the archaeological heritage park that covers Terceira's wreck sites; no diving permit or fee applies to the cave, though a separate visitor limit governs landing on the islet itself. Octopus Diving Center, based in Praia da Vitória, runs trips to the cave, and it is worth asking any Terceira operator you book with whether they still do too.