(CNN)The Forbidden Island.
It sure sounds like a marketing ploy to justify the high price and long sea voyage associated with a full-day trip offered by three scuba shops on the Hawaiian island of Kauai.
As far as most visitors know, the Hawaiian Islands stretch from Hawaii at the southeast end of the chain to Kauai at the northwest end.
However the uninhabited Northwestern Hawaiian Islands continue the archipelago nearly to the Bering Sea.
In between is a lesser-known island that lies 18 miles west of Kauai, but 138 miles southeast of the closest Northwestern Island. Privately owned Ni'ihau measures six by 19 miles, with fewer than 200 residents.
The native Hawaiian residents speak Hawaiian as their primary language.
To preserve their privacy and way of life, the brothers who own the island have severely restricted access by outsiders.
Until recently an invitation to Ni'ihau was one of the rarest commodities in Hawaii.
Even Mick Jagger couldn't get a permit to land his helicopter.
Recently the owners started running some helicopter tours, but these don't include any interaction with residents. Nor do they include any opportunities to scuba dive on the fabled reefs around Ni'ihau.
For this, people need to take a boat from Kauai.
While the island is no longer completely "forbidden," the journey across the Kaulakahi Channel is still intimidating.
The 33-mile voyage from a dock on the south shore of Kauai to Lehua Rock, on the north side of Ni'ihau, isn't bad.
It's the return trip facing into trade winds that tests sea legs. While limited access, distance and challenging logistics may add an air of exclusivity to a destination, they don't necessarily confer any real superiority.
In this case, however, experienced divers offer assurances that the dives around Ni'ihau and neighboring Lehua Rock are among the best in Hawaii.
Various types of whales, dolphins, whale sharks, big tuna and other pelagic life may be encountered during the outbound 2.5-hour boat ride.
Upon arrival at Lehua, it's a very different world from the rich, green mountains of Kauai, which receive some of the highest rainfall on earth.
That rainwater drains to the sea, bringing sediment to cloud the water and fertilize the growth of algae.
Ni'ihau and Lehua, by contrast, are extremely arid.
As a result, their waters are blue and clear, with frequently astounding underwater visibility.
The underwater seascape is also dramatically different, with vertical walls and spires that plunge precipitously to depths of hundreds of feet.
With open ocean so close at hand, there's a strong chance of seeing large sharks, rays and other denizens of the deep that get the adrenaline pumping.
The dive guides tell me they never miss an opportunity to work one of these trips.
"I've dived all over the world," Gaither Rosser of Bubbles Below Kauai says, "and Ni'ihau is among my top three dive sites in the world that are just spectacular."
The distance is a deterrent to fishers and aquarium fish collectors, so there's an abundance of reef fish.
On my first dive at Lehua, I spot the only giant Hawaiian grouper I've seen in 20 years of diving Hawaii.
"There's a gamut of rare fish," says Captain Ryan Ferry of Seasport Divers. "Some species are only really seen in the Northwestern Islands and occasionally around Kauai."
Another highlight is an animal so rare it's almost ghost-like.
"Your average person," Ferry says, "if you tell them there's a seal in Hawaii, they wouldn't believe you."
The Hawaiian monk seal, found only in Hawaii, is one of the most endangered marine mammals in the world.
"It's almost like going on a hike and seeing a wild panda," Ferry continues. "Lehua Rock is the only place in the world where you can reliably dive with Hawaiian monk seals," says Chris Kaiakapu of Bubbles Below Kauai.
In several trips, I always see at least one grumpy-looking male seal guarding its territory on one or both of the first two dives.
The third dive is a "structure dive," devoted to wildlife-spotting and dramatic topography.
There are caves, tunnels, gorges and giant arches.
It's like being able to fly through Utah's Arches National Park with a James Bond rocket pack, defying gravity. Within the crevices of the structures hide lobsters, shrimp, nudibranchs and other dwellers of the darkness.
Afterward it's time to face a return trip that Seasport Divers promises "can be as famous as the diving," lasting 2.5 or 3.5 hours, depending on weather and boat size.
"Sometimes we take a wrong turn and end up in the Bering Sea!" says Captain Ryan.
The jolts and drenchings are soon forgotten when we enter the calm lee of Kauai, filled with memories of unforgettable underwater experiences.