It has now been 100 days since an oil rig exploded in the Gulf of Mexico, triggering the worst offshore oil spill in U.S. history.

The oil slick appears to be clearing up faster than expected, but concerns remain about oil below the surface.

The head of the National Oceanic and Atmospheric Administration, Jane Lubchenco, says a significant amount of oil has been dispersed and been biodegraded by naturally occurring bacteria. She said Tuesday scientists are trying to determine where the remaining oil is and what areas it is likely to impact.

The incoming head of oil giant BP, the company responsible for cleaning up the disaster, says he does not "underestimate the nature of the task ahead."  

Robert Dudley, who will replace Tony Hayward as CEO of BP in October, says sealing the ruptured oil well permanently is his top priority.

BP confirmed Tuesday that Hayward would be stepping down from his post.  The announcement was made as the London-based company reported a record loss of $17 billion in the second quarter of the year due to the devastating spill.  

The Washington Post newspaper says U.S. investigators are launching a criminal probe into whether close ties between BP and U.S. federal regulators contributed to the disaster.

The paper says the "BP squad" will also examine whether BP officials made false statements to regulators, obstructed justice or falsified test results for devices such as the rig's failed blowout preventer.

Retired Coast Guard Admiral Thad Allen, the official in charge of the U.S. government's oil spill response, says efforts to seal the broken well are focused on pumping heavy mud and then cement into the top of the well.  He says that operation should begin August 2.