An oil rig caught fire and then sank the BP Deepwater Horizon into the ocean waters last week (APR.2010) in the Gulf of Mexico. An estimated 42,000 gallons of oil per day are leaking from the oil well.
When the oil rig exploded, at first the only oil evident in the water was that which had been on the rig itself. But, after that, it was discovered that water was also leaking from a broken pipe that led up to the rig from the well 5,000 feet below sea level. In order to shut down the existing leaking well successfully, a relief well will have to be drilled.
The graphic below explains what is happening on the ocean floor and how the relief well will work.
Source: nola