Rain and wind lash at my Gore-Tex as I make my way up a trail on the island of Ursholmen toward Sweden's westernmost lighthouse. On a fine day, I'd linger to take in the solitude, the salty air, and the stark outer-archipelago scenery. But today the sea is dark and opaque, and the land—a treeless expanse of gray rock and brown and green grasses, with a cluster of decaying buildings—feels like a place where human visitors are tolerated, but only just. Looking seaward, I find it hard to fathom the unseen wonderland beneath the dreary surface.

"I can only think of one experience which might exceed in interest a few hours spent underwater, and that would be a journey to Mars." — William Beebe, deep-sea explorer

It's quite a change from yesterday, when I biked and hiked under sunny skies on North and South Koster, the archipelago's main islands. I wandered through woodlands and fields to secluded inlets and stood atop the rocky heights of the Valfjäll lookout. That evening, in the Scandinavian summer's lingering twilight, a thunderstorm roared in, rattling the windows of my rustic cabin and illuminating the northern sky with pulses of lightning.

These islands, it turns out, are most treasured because of something I can't see: an underwater environment so diverse and unusual that the Swedish government recently enshrined the Kosterhavet Marine National Park—the nation's first such sanctuary. A deep undersea channel to the east abuts shallow waters, creating a string of habitats where intertidal oysters and mussels thrive in proximity to corals, harbor seals, and even an occasional whale.

Back on the mainland on this dark afternoon, I drive to the aquarium at the Sven Lovén Centre for Marine Sciences. As I gaze through the glass at creatures both familiar and bizarre—skinny pipefish, Technicolor sea stars, soccer-ball sponges—I get a glimpse of the hidden richness that thrives here.