For three decades, filmmaker James Cameron has vividly rendered alien worlds.

On Monday, ocean explorer James Cameron visited one: the bottom of the sea.

Nine hours after completing a historic solo dive to the deepest slice of the ocean floor, Cameron described his “very surreal day” in the language of an astronaut.

“When I came down, landed, it was very, very soft, almost gelatinous, a flat plain, almost featureless plain, and it just went out of sight as far as I could see,” Cameron said when he got back to the mega yacht Octopus, owned by his friend, Microsoft co-founder Paul G. Allen.

“The impression to me, it was very lunar, a very desolate place, very isolated,” Cameron said. “My feeling was one of complete isolation from all of humanity. I felt like I had literally, in the space of one day, gone to another planet and come back.”

Cameron’s adventure began at midnight, a day late because of choppy seas. He prepped the Deepsea Challenger for a few hours, then at 5:15 a.m. his time Monday began a quick but uncomfortable descent.

His goal when he got to the bottom of the Mariana Trench in the western Pacific Ocean — a depth of 35,576 feet — was to search for life. “We’d all like to think there are giant squid and sea monsters down there,” he said. There weren’t. He saw no fish, either. He found “nothing larger than about an inch across” — shrimplike scavengers called amphipods.

Cameron described extremes of pressure and temperature like those experienced by space travelers. He scrunched himself into a tiny metal pilot sphere “kind of like a Mercury astronaut,” the first American space travelers.

Electronics packed into the pilot sphere quickly heated the interior to 100 degrees. But after “one or two minutes” of plunging, the ocean temperature dropped to 36 degrees. The cold seeped in to his head and feet, which were pressed against metal; the core of his body stayed warm.

With high-definition 3-D cameras filming, Cameron wanted to grab rock and sediment samples with the sub’s hydraulic arms. But soon after taking his first sample, a swirl of hydraulic fluid drifted past his porthole. The arm was dead, its liquid lines crushed by pressure. A bit of the sediment survived inside a container and made it to the surface, but Cameron had to abandon plans to grab extensive samples on the quest, which was sponsored by the National Geographic Society.

He kept filming as he steered the lime-green “vertical submarine” up the sloping sea bottom. He searched for rock outcroppings that might hold exotic communities of tubeworms and other oddities, but he found none.

Problems mounted, and Cameron cut the planned six hours of bottom time to three. “I lost a lot of thrusters,” Cameron said immediately after the dive, according to a recording made at the time. “I lost the whole starboard side. That’s when I decided to come up. I couldn’t go any further — I was just spinning in a circle.”

Human deep-ocean exploration, too, has been spinning in circles since January 1960, when the only other piloted trip to this deepest spot in the ocean, called the Challenger Deep, occurred. U.S. Navy Lt. Don Walsh and Swiss oceanographer Jacques Piccard touched bottom in the Trieste, a 150-ton blimplike behemoth. But with just 20 minutes on the bottom — and with their view obscured by silt — that trip was a technological rather than scientific success.

Piccard died in 2008, but Walsh, whom Cameron calls a mentor, was on one of the support ships. “Don was here to sort of bless the journey, so to speak.” They’re now the only surviving members of a “unique club,” Cameron said.

Cameron sees his journey as the beginning of a long-term campaign to study the deepest slices of the ocean, the “hadal depths.” These trenches make up some 3 percent of the sea bottom — an area as vast as the continental United States — but remain almost entirely unexplored.

“The last frontier that really exists for us that is unmapped and unseen is the deep ocean,” Cameron said as the Octopus sped toward Guam, where a jet awaited to spirit the writer and director of “Avatar” to London for the premiere of “Titanic 3-D” on Tuesday. The day-long delay might make him miss the red-carpet event.

Cameron and Australian engineer Ron Allum designed the Deepsea Challenger to be light and fast. The craft weighs 12 tons, its core a high-tech “syntactic foam” invented by Allum for this very trip. Even so, the eight-ton-per-square-inch pressure at depth squeezed the craft by three inches.

Cameron said he and Allum will make “three or four” more dives in the coming weeks. The science team “wants samples, they want rocks” to study the geology of the trench, a slash where one of the Earth’s huge tectonic plates dives under another. Slippage of the plates in such zones can trigger huge earthquakes and tsunamis, such as the 9.0 magnitude quake that originated in an ocean trench off the east coast of Japan last March.

“It’s time to finally open up this frontier to science,” Cameron said.

But this kind of science is expensive, and government research budgets are stagnant or declining around the world, said Tony Haymet, director of the Scripps Institution of Oceanography. He’s hopeful that Cameron’s ad­ven­ture attracts more money — private or public — for exploring the deep. “We want to go there repeatedly for 10 years,” Haymet said. He sees exploring the deep as akin to exploring Mars: Robots will do most of the work, but humans’ journeys will fire the public imagination.

Imagination is, of course, in ample supply with Cameron: He’s dreamed of landing at the deepest point in the ocean since at least 2003, when he was filming his sea-creature documentary “Aliens of the Deep.” He asked that expedition’s science coordinator, Christina Reed, to put together a dossier on everything known about the Challenger Deep. “He gets projects in his head, and he mulls them over for a long time,” Reed said. “I got the impression that he was very serious about it and also very secretive.”

In 2005, Cameron and his engineers began developing the Deepsea Challenger, out of public view. But in February, the expedition was thrust into the open when noted ocean filmmakers Mike deGruy and Andrew Wight died in a helicopter crash while heading out to film a test dive of the minisub.

On Monday, Cameron said he was “sick at heart” after the crash; he considered scrapping the expedition. But the families of the two men encouraged him to go on. His own drive did, too.

The journey was a culmination of a lifelong dream for the 57-year-old Academy Award- winning director. As a boy growing up in landlocked rural Canada, he was enthralled by the television specials of ocean pioneer Jacques Cousteau, whose vivid underwater footage brought the ocean into millions of living rooms just as astronauts were preparing to step on the moon. At 16, Cameron became a scuba diver, his fascination with the deep fixed.

In 1995, Cameron’s dream of diving to the wreck of the Titanic — which sank 100 years ago next month — came true as he began filming the blockbuster movie about the ship’s sinking. At 12,600 feet down in the north Atlantic, the Titanic was as deep as he’d ever get, Cameron recalled thinking at the time.

His dive Monday took him nearly three times deeper.

During the descent, as Cameron watched the sub’s depth readings soar to “incredible numbers,” he felt “butterflies” of anxiety, fear, adventure, excitement.

“I was thinking like an astronaut, like a pilot: I’ve got to do this thing and not mess up. The fear goes away,” he said.

At the bottom, he reminded himself of a lesson he learned at the Titanic: He turned away from the monitors and touch screens and cameras stuffed into the cockpit and spent a pure moment “bearing witness, a sense of just being here.”

Alone, in the deepest dark, Cameron peered out a small window and soaked in the desolate wonder of an alien realm.