Their meaning is a mystery, their creators are long dead, and no one knows how many there are. The hundreds of thousands of engravings etched into boulders and cliffs on a remote desert peninsula in Australia form the world's largest collection of rock art.
Now there are fears that the planned expansion of an industrial site could destroy many of the Aboriginal engravings.
The petroglyphs, which depict human figures, abstract motifs and kangaroos, emus, and the extinct Tasmanian tiger, are scattered across the Burrup peninsula.
They were carved in granite more than 20,000 years ago by generations of Aboriginal hunter-gatherers attracted to the area by plentiful game and the caves' shelter.
The granite has ensured that the art is better preserved than carvings in soft sandstone found elsewhere in Australia. But the art shares the 12-mile peninsula with huge natural-gas plants and iron-ore shipment facilities. The expansion could destroy hundreds or thousands of carvings, conservationists say, and would be cultural vandalism akin to the destruction of Afghanistan's 1,600-year-old Bamiyan Buddhas.
"Can you imagine the Brits digging up Stonehenge? says Tom Perrigo, the director of the National Trust of Western Australia. "It wouldn't happen, and yet this rock art is equivalent in significance."
Last year, the World Monuments Fund included the peninsula in the world's 100 most endangered sites. The government of Western Australia said this month that it will try to block a bid to have the art placed on the National Heritage list, giving it more protection. It said the listing would have "grave consequences" for the expansion.
"We're trying hard to work with industry and the Aboriginal community to minimize any impact on individual sites while also managing the multibillion-dollar industry in the area," the government stated. Much of the development would be away from the gullies and uplands where the petroglyphs are, it said.
Woodside Energy, which has a stake in the peninsula, wants to develop a gas processing plant on the west coast, where there is significant rock art. It concedes that some carvings will be destroyed. But "sixty percent of the peninsula has already been set aside for conservation," says corporate-affairs manager Rob Millhouse. "There are carvings all over the peninsula. It's impossible to preserve every one."
Industrial development first started in the mineral-rich area in the 1970s. The National Trust estimates that up to 10,000 petroglyphs have been destroyed since then.
Beyond rock art, there are millennia-old standing stones, stone circles, and grinding stones. The tribe whose ancestors created them were wiped out in the 19th century by disease and clashes with colonial police.
About 10 percent of the peninsula has been built on and there are plans to develop a further 30 percent. "This is undoubtedly the greatest concentration of petroglyphs in the world," said Ken Mulvaney, president of the Australian Rock Art Research Association. "The state wants to develop 40 percent of the peninsula - that's like Sydney deciding to chop away part of the Opera House."
Conservationists are lobbying federal environment minister Ian Campbell, who will decide in September on the listing issue.
"We are not against development - there are areas ... where development could occur," Mr. Perrigo says. "[W]e're saying we now need to draw a line in the sand."