A salt dome is nature's version of Fort Knox -- a geological phenomenon that can store anything from table salt to nuclear waste.
Sheathed by a solid, dry, virtually impermeable wall of salt and covered with dirt, it resembles a mountain buried to its summit.
Last week, this security was breached when a Texaco drilling rig apparently accidentally pierced the shield of the dome at Jefferson Island, La., creating a crater that drained the lake above it and sucked in barges, tug boats and an oil rig.
The bizarre accident was enough to give nervous jitters to some who advocate using these natural underground vaults as storage space for oil -- as they already are being used in Louisiana and Texas -- or for nuclear and other hazardous wastes. But others are unperturbed. Said William Huls, former secretary of Louisiana's Department of Natural Resources: "It wouldn't happen again in 400 years."
Even so, after the Jefferson Island accident, Sen. J. Bennett Johnston (D-La.) called on Energy Secretary Charles Duncan to reexamine the use of salt domes as storehouses for oil and nuclear wastes.
The structures seem ideally suited to storage uses. Some extend as far down as 10,000 feet, gradually widening like a big bell.
Two salt domes in northwestern Louisiana have been studied as possible storage sites for nuclear waste. Dr. Joseph D. Martinez, director of Louisiana State University's Institute for Environmental Studies, said last week's accident had "almost nil implications" for use of salt domes as storehouses for this hazardous material or for oil.
Hollowed-out domes in south Louisiana and Texas already hold stocks of oil for the Strategic Petroleum Reserve program, and domes southeast of Jefferson Island are being hollowed out to store oil for the superport being built near Grand Isle, La. After the Jefferson Island accident, superport officials said they control land use above their domes, and they would not permit drilling into the salt formations.
If nuclear wastes were moved into the Louisiana domes, Martinez said, the federal government, which would own the land above the domes, would not permit any drilling there.
However, salt domes are not without other risks. In 1978, at a dome used to store oil for the strategic reserve, an explosion killed one workman and seriously injured another. Ten years before that accident, a fire at a salt mine killed 21 miners.
The only salt dome accident similar to the Jefferson Island cave-in occurred in 1965 when water from a quarry above the salt mine seeped into the mine and flooded it. But Martinez said most salt domes do not have bodies of water above them.
The salt domes, which abound in south Louisiana and Texas, were formed about 130 million years ago after a prehistoric sea evaporated. The salt remained, gradually becoming covered with sediment deposited by rivers like the Mississippi.
But the salt did not stay down. It rose wherever it found a weak spot in the sediment above it -- like wet sand rising between the fingers of an outspread hand.
This upward movement began about 70 million years ago, and the domes still move upward at an annual rate of a few millimeters. The results are outcroppings that became known as islands, even though most really were not islands but pieces of land that protruded from the flat landscape.
The salt, welling up through the earth, creates deformities or fractures in the rock layers, and those fractures frequently "trap" oil, gas and valuable minerals -- thus the concern that the domes might be popular sites for drilling.
To mine the salt inside, a shaft is sunk at the top of the mound, and caverns are dug that get bigger as the shaft gets deeper. These chambers are supported by pillars of salt, deliberately left for that purpose during mining, which give each cavern the appearance of a crude, cold Gothic cathedral.