Largest Oil Spill in U.S. History Continues Despite Efforts to Stop the Leak

Officials were hopeful about a technique called "top kill" involving heavy mud to plug the leak, but after three days, British Petroleum, known as BP, announced the effort failed.

Obama administration reacts to criticism

	President Obama addressed criticism about his administration's reaction to the oil spill and later traveled to the Gulf to monitor the situation.

Meanwhile, President Barack Obama announced he would continue a ban on new offshore drilling permits for the next six months. He also suspended oil exploration efforts near Alaska and Virginia and at 33 sites in the Gulf of Mexico.

The president also responded to criticism that his administration had acted too slowly in handling the oil spill catastrophe, stating that he has been "singularly focused" on cleaning up the spill from the beginning. He added that he will hold BP "fully accountable" for cleaning up and paying for the fallout from the spill.

The Minerals Management Service (MMS), the government office responsible for monitoring offshore drilling, has also been accused of responding too slowly to the spill. The head of the MMS resigned after only 10 months on the job amid mounting criticism from the public.

A poll conducted by Gallup and USA Today found that six out of 10 adults think the federal government is doing a "poor" or "very poor" job of handling the spill. Most adults polled -- 53 percent -- say the same about President Obama.

How does the "top kill" technique work?

	The "top kill" technique involves pumping viscous "mud" into the oil well to counter the pressure of the oil.

Stopping the oil leak has been complicated by the location of the leaking well -- about 5,000 feet, or nearly a mile, below the Gulf surface.

BP used underwater robots to attach long hoses to the blow-out preventer (a large valve) that originally failed to keep the oil in. Those hoses were connected to a ship on the surface.

From the ship, workers put about 50,000 gallons of heavy drilling mud through the hoses to try to plug up the hole.

However, on Saturday, BP announced that the "top kill" did not work, and the company is considering other options such as a large funnel-like contraption to try to capture at least some of the leaking oil.

Oil is coming ashore in different forms

	Tar balls have begun washing up on beaches along the Louisiana coast.

Estimates of how much oil has leaked into the Gulf vary widely, and no one has been able to get a fully accurate measurement thus far.

By measuring both the oil coming out of the well and the oil on the surface of the water, a team of researchers from the U.S. Geological Survey found that between 12,000 and 19,000 gallons of oil have been gushing out per day -- more than BP's original estimate of 5,000 gallons.

Even the researchers' most conservative estimate surpasses the amount leaked in the 1989 Exxon Valdez oil spill, previously the largest in U.S. history.

Oil from the spill is washing ashore along the Louisiana coast, in the form of both an oil slick on the top of the water and large balls of tar.

Lighter particles float to the top and form a slick on the top of the water, while the heavier particles swirl under the surface and bind together into tar balls with the help of sunlight, seawater and microbes.

Tar balls are generally easier to clean up because they can be picked up by machines or people cleaning up the beach. But, they are just as toxic as liquid crude oil and also endanger wildlife.

To keep floating oil from reaching delicate wetlands, workers have surrounded islands and coastlines with miles of floating boom, or barriers. Some oil has passed those barriers, however, and rescue teams are hard at work finding and cleaning oil-soaked animals such as dolphins, turtles and birds.

Cleanup will be a lengthy process

	The Exxon Valdez spill, previously the largest in U.S. history, officially took three years to clean up, but fishermen still find oil in the area.

Cleaning up the spill will likely take decades and wildlife ecosystems near the Gulf of Mexico will suffer for years.

Fishermen in Alaska's Prince William Sound, where the 1989 Exxon Valdez spill happened, still find oil under rocks on shore.

Oil degrades faster in warmer water, so scientists hope the process won't take as long in the warmer Gulf waters.

Although the largest oil spills get the most attention, smaller ones happen frequently in rivers, lakes and tributaries around the country.

In 2000, 140,000 gallons of oil leaked into Maryland's Patuxent River, which feeds into the Chesapeake Bay. Ten years later, scientists say most of the area's wildlife has recovered, but traces of oil can still be found in the river and the bay.