The kayakers negotiate the crease in the rock and follow a narrow passageway into a spacious cavern, where they pause, as if with great trepidation.
But it's mostly just awe.
No more daylight. No trees or grass fields waving in the wind that blows across Santa Cruz Island. No pelicans soaring beneath a bright blue sky. No bright blue sky.
Just enough penetrating sunlight to illuminate the watery fringes of this great chamber an emerald green.
Overhead is a rock ceiling smoothed by waves, dripping eternally atop the shifting blackness. Below is the blackness, home undoubtedly to creatures fond of such environs, whether with savage teeth or long tentacles no one can be sure.
Imaginations run wild while exploring Santa Cruz Island's craggy shoreline -- especially its many caves -- in large part because of its storied past.
The Chumash Indians, according to legend, emerged here from seeds of a magic plant by the Earth goddess Hutash, and were provided fire via a lightning bolt by her husband, Sky Snake, or the Milky Way.
European explorers visited. So did missionaries and fur traders, who probed these very caves, seeking sea otters, seals and sea lions, which they hunted to near extinction.
Today's explorers come simply with a thirst for adventure.
"I've seen these islands from a plane. I've seen them hiking and diving. I've read about them and I write about them," says Rocio Lozano, a representative of the Channel Islands National Marine Sanctuary, on her first kayaking field trip. "But in a kayak, you're one-to-one. It's much more intimate, and you see a lot more detail."
Santa Cruz Island, the largest of the Channel Islands at 22 miles long and up to six miles wide, is a small replica of pre-development California. Its mountain ranges are split by a vast central valley. Its distinctive flora and fauna are protected by the Nature Conservancy and Channel Islands National Park.
The nutrient-rich waters surrounding the five northern islands are kept pristine through sanctuary protection and frequented by several types of whales and other marine mammals. Visitors are often accompanied by dolphins playing in the wake of the Island Packers' transport boat.
Kayakers are few, compared with hikers and campers, but are looked on with envy as they depart the big boat, climb into their little boats and paddle off. No great skill is required; almost anyone who can afford the $160 excursion can do it.
At this time of year, says Channel Islands Kayak Center guide Chuck Graham, "You can be alone in your kayak and all you're going to hear is water lapping against the island, or the whisper of the oystercatcher [a common shorebird], or the barking of a sea lion."
Graham's group, including Lozano and two others, left the Scorpion Ranch landing on a sunny, breezy morning, traveling silently over amber kelp forests under towering cliffs.
They journeyed through a long gap carved between a rocky promontory resembling an elephant's head and trunk. They entered small caverns, which seemed to put them in the soul of the island, such were the whooshing sounds caused by the surge of tide and air.
In the back of expansive Harbor Seal Cave is a small beach that provides shelter for pinnipeds. Deep into the cave the kayakers went, marveling at the coolness and solitude, and out they quickly journeyed when three small seals were discovered on the beach, eyeballing them with concern.
Nearby is an even larger chamber, Cavern Point Cave, extending nearly 400 feet and with dank shores and ledges that are nesting sites for sea birds. Towering overhead is the Cavern Point bluff, reachable via a short hike and, as the paddlers learned later, affording breathtaking views.
Along the coast, in the shallows, the water is gin-clear. Lobsters can sometimes be seen scurrying along the bottom. Stingrays glide over the sand like alien spacecraft.
"We don't have anything like this in the South," says Brian Thompson, 40, a visitor from Memphis.
Graham's clients have negotiated nearly a dozen caves -- including an eerie passage named Shark's Tooth -- and now they've entered the ever-popular Green Room within Scorpion Rock. Their excursion is near an end so they slow their pace.
Finally, they paddle through the exit and sunlight washes over them like a wave. A sea lion shoots them a glance. Pelicans notice and some take flight, beneath a cloudless sky that never looked so blue.