Call it oil spill interruptus. A day after Coast Guard Admiral Thad W. Allen—on the advice of academic and government scientists led by Energy Secretary Steven Chu—abruptly stopped a planned attempt to halt the flow of oil from the new containment cap and measure the integrity of the wellbore, the all-important test is now back on. “We plan to initiate the test this evening,” Allen told reporters in Houston at 4 PM today, just after returning from one last visit to the site of the leak 40 miles south of the coast of Louisiana. “I was gung-ho for the test and I remains so now”—a statement that passes for high emotion from the usually taciturn admiral.

In fact, BP has already begun the process of closing off the flow of oil from the new, just-installed containment cap. The company has taken the Q4000 and Helix Producer—which had been diverting thousands of barrels of oil—offline as part of the preparation. If you go to BP’s live underwater camera feeds, you can see that crude has stopped flowing from the top of the new containment cap, known as the 3 ram stack. That’s the first step. If they haven’t already by the time you’re reading this, BP will soon close the kill line on the stack—another vent where oil had been escaping—and then eventually squeeze the choke line, the last escape for the oil. While first two vents are open-shut, the choke line can be, as the name goes, choked—closed gradually, a process that should give the engineers time to gauge any potential problems as pressure gathers in the cap and the well. And when the choke line is fully closed—which should be fairly soon—the integrity test will really begin.

As we wrote before, BP will be looking to gauge how much pressure is inside the well. If the pressure is high and remains stable—Allen said about 8000 to 9000 pounds per sq. inch (psi)—that means that the oil is being contained within the wellbore and that it must be in decent physical shape despite the trauma that destroyed the Deepwater Horizon. That would also mean that BP might be able to keep the containment cap’s vents shut, effectively sealing off the well—and it would be good news for the drilling of the relief wells, because its simpler for drillers to deal with an intact wellbore.

If the pressure inside the wellbore falls and remains low, however, it means that the wellbore has likely been damaged and that oil is escaping elsewhere—into the layers of rock around the oil deposit, and possibly into the Gulf via the seafloor. That would mean capping the well early would be impossible, forcing BP to go back to trying to contain the flow as best they can by diverting the oil to ships on the surface. It would also add an extra layer of challenge to the relief wells, which are still scheduled to be completed by the end of July or early August.

Essentially the procedure for the integrity tests is no different than it was a day ago—so why did Allen halt the process yesterday, and why did he give the go-ahead today? I’ll let him answer: “An overabundance of caution.” Though BP was ready to go forward and close the containment cap yesterday, Chu and his fellow scientists wanted more data. They wanted to be absolutely sure that in letting pressure build up in the wellbore, they wouldn’t inadvertently damage it and cause what Allen called “irreversible leakage outside the well.”

The word “irreversible” is rarely used to describe good things (irreversible joy?), and that’s the case here. If you think it’s been difficult to control the oil spill as the crude flows through the remnants of the Deepwater Horizons blowout preventer—sitting on the floor of the ocean—imagine trying to control it if the oil starts bleeding through any number of openings on the seafloor. It’s the difference between trying to save a patient who has been shot once, and one who has been peppered by bullets.

So Chu and his team asked for more time to go over the data—specifically the seismic surveys of the rock around the wellbore that were done earlier this week. Allen says they wanted to know whether the surrounding geology was of the kind that might be vulnerable to this sort of catastrophic leak—and whether the integrity test could unwittingly cause that cataclysm to happen. “In the interests of the safety of people and the environment, we needed to take a 24-hour break to make sure we got this right,” Allen said this afternoon.

They did—Allen said the additional tests “removed the possibility of a negative event,” which is a statement one hopes he doesn’t live to regret. Now the integrity test is going forward. Which answers at least one question—ultimately, it is the U.S. government that is finally in charge of the containment procedures over the wellhead, as BP seemed perfectly ready to go forward with the process without more safety tests. BP’s lack of caution is a little bit worrying in its own right— and this afternoon BP vice president Kent Wells assured reporters that this is “a BP operation,” though as the test unfolds every six hours the company will take the pressure data and consult with Chu and his team on whether or not to continue.

(By the way, how cool is it that we have a Nobel Prize-winning energy secretary, one who isn’t just a political figurehead and can actually read a technical situation and make the right call. He’s like Spock—if Spock were a cabinet secretary.)

For now again, we’ll wait—and watch thanks to the feeds. If there is a problem with the wellbore and the pressure doesn’t rise, we should know within a few hours. If it comes, failure will come fast.

Update: Which may be exactly what is happening. BP found a leak on a line attached to one of the valves, and as of early Thursday morning was unable to close all the vents. The company says it is fixing the problem—but this is unlikely to be the last thing to go wrong.