Eons ago, an upheaval of the sea bed formed what over time would become part of Southern California.

A tropical climate prevailed there for millions of years until tectonic forces created a coastal mountain range that blocked the arrival of moisture from the ocean.

Plants and animals vanished. The ground turned to sand. And starting around the 1950s, it became the favored habitat for a new form of life — off-road enthusiasts.

The sandy region east of San Diego, known variously as Glamis Dunes, Imperial Dunes, and Algodones Dunes, covers an area of roughly 40 miles by six miles. It’s the largest dune ecosystem in California.

Most days, the remote area appears lifeless and empty. But on holiday weekends between Halloween and Easter, it’s swarmed by as many as 100,000 people towing dirt bikes, dune buggies, and every other manner of off-road vehicle.

As recently as a decade ago, the gatherings at Glamis were notorious for their Mad Max-style lawlessness, wild parties, and fast-riding, said John Cappa, an off-roading journalist based in Los Angeles.

“It was almost like the Wild West back then, but it’s nothing like that anymore,” he said. “I think the craziness basically stays in people’s camps now.”

That doesn’t mean environmentalists like it. The dune ecosystem is home to unique desert plants and animals that they say are threatened by the motorized vehicles. While part of the area has been cordoned off, some groups want off-roaders banished altogether.

The Bureau of Land Management, which oversees the park, has for the most part sided with the off-road community.

Many of the regulars at Glamis see the outings as a way of life, forged with friends and family not just on wheels but around campfires and under the star-filled desert sky.

But the dunes aren’t for off-roaders only. The picturesque sandscape is popular as well with hikers and horse-back riders. Tour guides recommend going in the spring when the desert is festooned with wildflowers.

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