"This is an uncontrolled science experiment which is unprecedented one mile below the sea," Rep. Edward Markey, D-Mass., said.
The fear is that the cap will not fit, and the oil will keep flowing, triggering an environmental domino effect.
"The whole face of the coast line could be impacted," Ed Overton, professor emeritus at Louisiana State University School of Coast and Environment, said.
The Coast Guard and BP are preparing for a massive cleanup effort that could last until autumn. They will begin redeploying their army of 20,000 workers to tent cities and so-called "floatels" near vulnerable areas, so workers can be dispatched quickly to fight oil as it rolls ashore.
As BP attempts the latest fix, the Obama administration has ordered the company to continue drilling those two relief wells that are not expected to be ready for months.
BP will have to drill two miles beneath the surface of the earth and hit a target approximately the size of a dinner plate.
"They are looking at some technologies, some camera technology that they will use to locate the pipe underground, but obviously we wanted to make sure there were redundancies because we need to get this thing stopped," White House Energy Adviser Carol Browner told "Good Morning America." "We've always understood that the relief well was the way to permanently stop it."