An Exxon Mobil pipeline ruptured Friday in Montana, spilling up to 1,000 barrels (42,000 gallons) of oil into the swollen Yellowstone River before the company could shut off the spigot. After initially saying the spill only sullied about 10 miles of the pristine waterway, about 100 miles downstream from Yellowstone National Park, Exxon on Monday acknowledged that the damage could be much more extensive. More than 280 people from Exxon, federal agencies, and the Clean Harbors environmental waste company are in Billings, Mont., to work on the cleanup effort. How bad will this "disastrous" spill be? Here, a brief guide:

What caused the spill?
State and federal agencies are still trying to determine what damaged the 20-year-old Silvertip pipeline, which usually carries 40,000 barrels of crude a day from Silver Tip, Mont., to Billings. The working theory is that seasonal flooding uncovered the pipeline, which at last check was buried five to eight feet below the riverbed, and that a tree trunk or some other large piece of debris opened a crack in the pipe. 

What kind of damage is it causing?
Oil has been reported at least 100 miles downstream, and could reach to where the Yellowstone empties into the Missouri River. As the waters recede from Saturday's flood-level highs, ranchers and others with land along the river are reporting brown patches of oil on their land. Wildlife officials say the oil will kill some of the area's birds, fish, and other wildlife. "My biggest concern is those 1,000 barrels," says Gov. Brian Schweitzer (D). "You cannot dump (that much oil) into a pristine trout stream without causing damage to the fisheries."

How's the cleanup going?
Exxon and cleanup workers have deployed 48,000 feet of absorbent boom and 2,300 absorbent pads to soak up the oil, but the boat-unfriendly high waters and swift currents are keeping workers off the river itself. And although the rushing water is breaking up the toxic oil faster than if the Yellowstone were dammed, it also means the environmental damage will spread farther downstream. "It could take years to really understand the impact of the spill," says ecologist Charles Preston.

Who's going to pay for the cleanup?
Exxon is on the hook for the cleanup costs, and Exxon Mobil Pipeline Co. President Gary Pruessing says the company will do "whatever is necessary" to find and mop up oil. Gov. Schweitzer, a soil scientist, agrees that Exxon should pick up the tab. But "everybody in Montana will work hard until this is done," he added. "We'll be on it like a stink on a skunk."

Sources: APBillings Gazette, CNN, Los Angeles Times