You hear them before you see them, the cars that Larry Volk and his fellow speed freaks race over the Bonneville Salt Flats in western Utah. After a while, they become dots on the horizon; then, pretty soon, noisy blurs that cover a mile of track in as little as ten seconds.
This famous patch of desert, wide, perfectly flat, and covered by a smooth layer of white salt, has been part of motor racing history since the 1930s, when Sir Malcolm Campbell appeared on the scene, with his leather hat, Biggles goggles and famous car Blue Bird. On his first visit here, he became the first man to drive a vehicle at more than 300mph.
Today, the Bonneville Salt Flats remain home to world-famous annual "hot-rodding" championships. Volk visits three times a year to race his pride and joy, a 1929 Ford roadster fitted with a 1200 horsepower Chevrolet engine. "It's hard work," he says. "Some days, holding the steering wheel is like going to a hula dance; but what a thrill!"
Thrills can be fleeting, though. And in recent years, Larry Volk and his peers have been faced with a nagging problem: though huge, vibrant crowds still travel from across the world to watch them race, there are growing fears for the future of these octane-fuelled high-speed sporting events.
To see why, you only have to walk across the flats, which have provided a unique setting for such films as Independence Day, Pirates of the Caribbean and Tree of Life, along with countless TV and print adverts. Every now and then, beneath the photogenic white surface, a patch of dirty brown will appear.
This is mud. And its appearance illustrates a sad fact: in recent years, the smooth, fast salt cover which is so essential to the business of setting land speed records has begun to disappear. "We started noticing the deterioration in the 1960s," says Volk, who first came to the flats, about two hours west of Salt Lake City, in 1958.
"Back in the day, the salt used to be two feet thick. Over the years, we noticed it starting to get thinner and buckling. In places now, it's only around half an inch thick."
That's thick enough to race on, but only just. Sometimes, in recent summers when the racing season comes around, the hot-rodding community has struggled to find the necessary seven-mile stretch of unblemished salt on which to build their temporary track, known as the Bonneville Speedway. The flats, originally 90,000 acres, are now a mere 30,000.
The blame, the racers say, lies with potash. Or more specifically, a potash mine run by a company called Intrepid Potash-Wendover, LLC, which sits a few miles south of their sporting playground, on the far side of Interstate 80 which connects Utah with Nevada. The mine uses a series of canals and pipelines to collect salt brine off the flats in the rainy winter season. Then, when summer comes around and the water evaporates, they process the remaining minerals to remove the potash, which is mostly used in fertilisers.
Over the years, the process has taken a heavy toll on the unique geological formation of the salt flats. Since 1963, when the mine was started, Bonneville has lost more than 55 million tonnes of salt, and about a million tonnes are still being extracted each year. As a result, geologists estimate that 18 inches of salt crust have completely vanished.
"When you remove brine from the flats in winter, you are draining off water with dissolved salt and minerals that would naturally be used for replenishment," explains Kenneth Kipp of the US Geological Survey, who has studied the disappearing salt. "That, naturally, affects the whole basin. The average decrease in the thickness of the salt over the entire area is in the order of 1 per cent per year. Over time, that sort of loss can of course add up."
Intrepid produces about 100,000 tonnes of potash a year, worth about $50m (£31m). In order to protect that income, it has in recent winters begun voluntarily pumping thousands of gallons of leftover brine back on to the speedway section of the flats.
Larry and his fellow hot-rodders are convinced that this replenishment operation works, and will over time reverse the decline of the flats and protect a unique environment which, eighty years after Malcolm Campbell, remains essential to the business of setting land speed records.
There is, however, one big problem: it is currently being done by Intrepid on a purely voluntary basis. Since mines and mining companies can often change hands, the racing community says it needs to be mandatory. They have formed a lobby group, the Save Our Salt Coalition, which is seeking a change in the law that will require all future operators to pump back the processed salt they have removed.
On paper, it's common sense. But in practice, things have proved complex. Like much of the rural West, the Bonneville Salt Flats are run by the US government's Bureau of Land Management (BLM). And the wheels of officialdom turn painfully slowly. "We told the Bureau about this problem 20 years ago," says Volk. "Since then, all they've said is that they're 'working on it'."
There have been studies, and committee meetings, and endless delays because of staff changes and budget cuts. "Only when we bring in lawyers and threaten legal action do they ever seem to make any kind of progress."
Last summer, Larry Volk and his peers were told to expect a final decision from the BLM on their compulsory pumping plan. But the summer came and went, and no decision was announced. The BLM's current position is anyone's guess. It didn't respond to multiple inquiries about the flats this week.
Meanwhile, the salt keeps getting thinner. "I love everything about this place," says Larry. "The cars, and the people, the thrill of watching someone setting a land speed record. My sons and my daughter have all raced here, and I hope my grandchildren will too. But you can't do that without a good layer of salt."