If BP fails to plug its ruptured offshore oil well, intense underground pressure would be enough to pump vast quantities of thick brown crude into the Gulf of Mexico for months, even years.
If even BP's backup plans fail, it would cause a pollution disaster "heretofore unseen by humanity," said one expert.
It is this rapidly accelerating realization that is giving BP's attempt Wednesday to cap the well new political and environmental urgency.
The worst-case scenario is hoped and believed to be a continued flow of 5,000 barrels per day, and by some estimates vastly more, until August, when BP completes "relief wells" to intercept the damaged well.
But, experts say, there are no sure things when operating equipment a mile under the water and 13,000 feet below the ocean floor.
Professor Tad Patzek, who heads the Department of Petroleum and Geosystems Engineering at the University of Texas-Austin, gives the relief well a 90 percent chance of success. But he'd rather not consider the other 10 percent.
"As a petroleum professional, I don't even admit the possibility that that might be possible," he said when asked about a failure to stop the flow. "That would be an environmental disaster of a caliber that was heretofore unseen by humanity."
Patzek estimates at least 20,000 barrels of oil and an equal amount of gas would flow daily for years from the reservoir, which he estimates to hold roughly 50 million to 100 million barrels.
"That is something that is not acceptable by any standards or measure," he said. "If BP cannot deal with the relief well, there will be somebody else that will, and that would happen sooner rather than later."
David Rensink, the incoming president of the American Association of Petroleum Geologists, similarly said that BP's Macondo prospect likely contains enough oil to keep flowing from the broken undersea well for a very long time.
"It would take years to deplete. You are talking about a reservoir that could have tens of millions of barrels of oil in it," he said in a statement provided by the group.
A spokeswoman for the American Petroleum Institute - the oil industry's biggest trade group - said the relief well effort is certain to succeed. "Will the first relief well work? We don't know. That is why they are doing multiple relief wells," said API's Cathy Landry.
BP on Wednesday began a "top kill" that involves pumping heavy muds into the well to block the flow. It is believed to be the most promising effort yet, but has never been tried at such depths. It is expected to be one to two days before it is clear whether the has measure worked.
The company is also weighing other plans if it fails, including another effort to place a containment dome over the leak.
Stopping the flow has become a consuming priority for the White House, which faces growing criticism even from some Democrats over the ongoing catastrophe. Pressure on the White House is matched, if nowhere else, at the sea floor.
Patzek said the amount of pressure in the oil reservoir is at least 12,000 pounds per square inch and perhaps higher, while the pressure at the sea floor is around 2,300 pounds per square inch. "These are immense pressures," he said, "by any standards."
President Barack Obama is playing an increasingly visible role; he's planning a press conference Thursday, where he is slated to announce new safeguards to accompany offshore development.
And Friday he travels to Louisiana, his second trip to the Gulf Coast since the spill began, to assess efforts to counter the disaster.