In the summer, the desert surrounding the Salton Sea in California seems to glow from the glare of the unrelenting sun. The temperature can climb to 110 degrees and higher, and layers of dust coat car tires and the legs of people braving the heat.

For most of the day, the sky stays a constant blue, the horizon punctuated by the orderly fronds of date palm trees grown on nearby farms. Almost 10 miles from the southeastern banks of the Salton Sea, about an hour-and-a-half drive from Palm Springs, another mark appears on the landscape, this one unruly and vivid: the rainbow hill of Salvation Mountain.

A large white cross sits on top of the sculpture, and, just underneath, among stripes of blue and swaths of green, the words “GOD IS LOVE” are spelled out in red and pink sculpted letters. Old, brightly colored paint cans line the perimeter, and scattered near the base of the hill are a few vehicles, nearly all of which have been painted with messages about Jesus and the Bible. The hill, made mostly of clay from the surrounding desert and donated paint, is 50 feet high and 150 feet wide.

Every year, thousands of tourists come to see Salvation Mountain, the sculpture that Leonard Knight spent the last decades of his life building by hand. They arrive by car and by bus, sometimes traveling across oceans to see Mr. Knight’s work.

Some are religious, while others are drawn more by the artwork’s aesthetic than the messages of faith running up and down and across the artwork. In a part of California that is often said to have been forgotten, the sculpture stands resolute, a reminder of one man’s ceaseless vision and the care that his friends and admirers have taken to try to stop it from succumbing to the harshness of the environment.

The sculpture was built on land that had once been a marine base outside of Niland, Calif. The abandoned camp is now better known as the location of Slab City, an unofficial community made up of vehicle squats. Some residents stay at the Slabs, as it is known, year round, but a majority are “snowbirds,” who come in the winter to escape colder climates up north. Many live off the grid, usually in vehicles or structures, sometimes built with found materials.

Mr. Knight arrived in the mid-1980s, according to Salvation Mountain Inc., an organization dedicated to preserving the sculpture. He grew up in Vermont before landing near Shelton, Neb., where he spent a few years trying to build a hot-air balloon. That balloon contained many early iterations of artwork that he eventually included in Salvation Mountain; it was made with brightly colored patches, and “GOD IS LOVE” was written on it in large letters. The balloon began to rot before he was able to fly it as he had hoped, and he put the project on hold to continue his journey to California.

His first version of the mountain, which he made with a mixture of sand and cement, collapsed around 1990, prompting him to use straw, hay and clay on his second try. Other parts of the mountain were reinforced with tires and telephone poles. He accepted donated paint, which he used liberally, creating thick layers that helped seal in his building materials.

Bringing Mr. Knight paint is how Bob Sims, 70, first got to know the mountain. A friend persuaded him to visit in the mid-1990s, and he estimates that over the next five years he brought Mr. Knight thousands of gallons of paint. Mr. Sims is now a member of the board of Salvation Mountain Inc.

“He was an amazing guy,” Mr. Sims said. “The most sincere person that you would ever meet.”

After Mr. Knight died in 2014, those close to him started to worry about the future of the sculpture, which needs constant upkeep. Over the last decade, parts of the mountain have collapsed or cracked, spurring Salvation Mountain Inc. to close off a path that visitors once used to climb to the top of the mountain. A walled-in section that Mr. Knight referred to as the “museum” has also been closed.

Since 2016, much of the repair work has fallen into the hands of Ron Malinowski, the caretaker for the sculpture and also a board member of Salvation Mountain Inc. Mr. Malinowski lives in a camp at the base of the mountain, and although he never met Mr. Knight, he feels that he’s learned the artist’s techniques by working on the mountain and by watching videos he found on YouTube, some of which document Mr. Knight’s process.

To fix parts of the mountain, “I have to deconstruct some of it,” Mr. Malinowski, 56, explained. “That’s how I learned a lot of the ways. And I watch all the videos and stuff. While he’s talking he’s working.”

Mr. Malinowski said he tried to emulate Mr. Knight’s techniques as closely as possible. Mr. Knight used to mix clay he dug from the desert for the sculpture in the bucket of a tractor, and so Mr. Malinowski does the same.

“I learned the stories from his mouth,” Mr. Malinowski said, referring to videos of Mr. Knight. Mr. Malinowski prefers to go barefoot when he can, and said that in the almost eight years since he started working on the mountain, he hasn’t worn sunscreen or sunglasses.

Along with Mr. Malinowski, a few docents — most of whom are Slab City residents — also work at Salvation Mountain, greeting guests and telling them about the history of the piece. One of those docents is Little Dribbling Gray Wolf, 57, who goes by Wolf and has worked at the mountain on and off for four years. He said that although the mountain had religious overtones, it was not meant to be an evangelist artwork.

“Leonard didn’t really consider himself to be a religious freak,” he said. “He didn’t consider himself to be an artist, he just wanted to put out a message, and that’s what he did. He had that focus to do it.”

The sculpture and Mr. Knight were featured in a scene of the 2007 movie “Into the Wild,” about a man who sets out into the wilderness, and a few music videos have been filmed at Salvation Mountain. More recently, though, Instagram and TikTok have fueled visitors, who come after seeing pictures and videos of the rainbow hill in the middle of the desert, baking under the sun.

“Nobody would come here,” Mr. Sims said of the time when he started visiting. “And the internet started to play a bigger role in it.”

TikTok is how James Hewitt, a Southern Baptist pastor from Jonesboro, Ark., ended up at Salvation Mountain in late June with his family. His daughter, Saraya Hewitt, had seen videos of the mountain, and while the family was on a trip in Palm Springs, they decided to drive out to see it.

“Honestly, from the videos, I thought it was going to be bigger,” Ms. Hewitt, 19, said. “But I still like it, and I still like all the colors, and all the floral painted onto it. I think it’s very pretty.”

In 2015, Salvation Mountain Inc. started talking to state land officials about buying the land the sculpture sits on, and last year it submitted an application for purchase and a deposit to the California State Lands Commission to buy 610 acres, which would include Salvation Mountain and Slab City. The application and the deposit were returned this July, and the organization said it hoped to resubmit a new proposal in the future, this time for only 40 acres of land.

This summer has been especially hard on the sculpture and for the surrounding community, as they have grappled with extreme weather — in an already extreme place. A sudden heat wave left Mr. Malinowski and Wolf sick, and a recent flash flood left some Slab City residents homeless and caused part of the sculpture to collapse.

“There’s just a handful of us that are trying to keep it together,” Wolf said. “It’s not been easy, and Mother Nature has been our worst enemy the past few years.”

Kitra Cahana contributed reporting.