In a word, surprise. Sinking beneath the swells into 52-degree (11°C) water, I'm floored by anemones that gleam as pink as bubble gum, as green as a lime snow cone, so orange I can almost taste the juicy pulp. Flatfish with their eyes topside wear mottled camouflage so spot-on that they hide in plain sight. Childlike gray seals nibble another diver's fin before rocketing out of reach. Their home, the Skelligs, is a pair of drip castles of red sandstone frosted with guano and feathered with thousands of gannets and puffins. The larger isle, Skellig Michael, rises over 700 feet (210 meters) above water and plunges another 160 (49 meters) below, merging with the continental shelf. To dive at the kelp-covered base of this rock with the legendary selkies as companions, then ride my exhaled bubbles to the surface, is a rare privilege.

It's a pair of warm flows--the North Atlantic Current from the Caribbean and the Shelf Edge Current from Portugal and France--creeping up Ireland's west and, indirectly, east coasts that keep winters and waters relatively mild and stable, a boon for marine life. Where I dove on the western side, animals, many at the extremes of their ranges, meet in clean temperate Atlantic waters that wash shores of varied rock types and topographies from exposed cliffs to sheltered coves. Habitats, as a result, are assorted and ample in the intertidal zone and on down the gently sloping shelf. Life explodes into every niche.

It doesn't take a deep dive to witness the sea's plenty. The walls of Valentia pier awaken at sunset: Busy invertebrate legs twitch and wave as animals skitter about, dancing in place or scrambling up vertical stages. Eye stalks mark time--poking out, drawing back, out, in. Crabs hoarding jellyfish scoot with clawfuls into crevices. In a plankton haze 20 feet (six meters) down, I move with slow kicks to the muffled clacking of shells and claws against bedrock bones.

On walls in deeper waters, gaudy anemone carpets give way to pan-size starfish and colonies of orange soft corals called dead man's fingers, which shoot out feathery white antennae to feed. A dogfish shark curls into a comma under my flashlight's glow; blennies raise eyebrows and giant conger eels poke heads, puppetlike, from the grottoes. (Much deeper and out of my reach, at nearly 2,000 feet [610 meters], lies a recent scientific discovery: cold-water coral gardens in a barely explored environment.) On my reluctant ascent I run a gloved hand through slippery kelp whips that nod in the current. Suddenly something pierces the sea surface and whooshes down in a stream of bubbles, stopping to eye me before beating a frantic departure. It's not a fleeing fish but a bird called a guillemot seeking a seafood meal--a fellow ocean visitor as astounded as I by creatures so unexpected.