Oil giant BP was cautiously optimistic today after finally winning control of the catastrophic Gulf of Mexico oil leak.
Engineers managed to fit a cap over a the runaway well that has been gushing since early spring, although no one was declaring victory just yet.
They will watch anxiously over the next day and a half to see if the cap holds.
The accomplishment was greeted with hope, high expectations - and, in many cases along the beleaguered US coastline, disbelief. But BP Chief Operating Officer Doug Suttles urged caution and warned the flow could resume.
"It's far from the finish line. ... It's not the time to celebrate," he said.
However for the first time since an explosion on the BP-leased Deepwater Horizon oil rig killed 11 workers on April 20 and unleashed the spill 5,000ft beneath the surface, no oil was flowing into the Gulf.
For now, engineers and scientists are monitoring the cap for pressure changes around the clock. High pressure is good because it shows there is only a single leak. Low pressure could mean more leaks further down in the well.
President Barack Obama, who has encouraged, cajoled and outright ordered BP to stop the leak, called the development "a positive sign".
But Mr Obama, whose political standing has taken a hit because of the spill and accusations of government inaction, cautioned that "we're still in the testing phase."
The worst case would be if the oil forced down into the bedrock ruptured the seafloor irreparably.
Leaks deep in the well bore might also be found, which would mean that oil would continue to flow into the Gulf. There is always the possibility of another explosion, either from too much pressure or from a previously unknown unstable piece of piping.
The drama that unfolded quietly in the darkness of deep water was a combination of trial, error, technology and luck. It came after weeks of failed attempts to stop the oil - everything from robotics to different capping techniques to stuffing the hole with mud and golf balls.
The week leading up to the moment when the oil stopped was a series of fitful starts and setbacks.
Robotic submarines working deep in the ocean removed a broken piece of pipe last weekend, at which point oil flowed unimpeded into the water.
That was followed by installation of a connector that sits on the spewing well bore - and by Monday the 75-ton metal cap, a stack of lines and valves, was latched onto the busted well.
After that engineers spent hours creating a map of the rock under the sea floor to spot potential dangers, like gas pockets. They also shut down two ships collecting oil above the sea to get an accurate reading on the pressure in the cap.
As the oil flowed up to the cap, increasing the pressure, two valves were shut off like light switches, and the third dialled down on a dimmer switch until it too was choked off.
It's not clear yet whether the oil will remain bottled in the cap, or whether BP will choose to use the new device to funnel the crude into four ships on the surface.
For nearly two months, the world's window into the disaster has been through a battery of BP cameras, known as the "spillcam".
The constant stream of spewing oil became a fixture on cable TV news and web feeds.
Thad Allen, the retired Coast Guard admiral overseeing the spill for the government, said officials are deciding as they go along whether to release oil into the water again. At the end of the 48-hour test it is possible oil will start to flow again - but, theoretically, in a controlled manner.
When the test is complete, more seafloor mapping will be done to detect any damage or leaks.
The saga has devastated BP, costing it billions in everything from cleanup to repair efforts to plunging stock prices.