Published: May 16, 2010 6:02 PM EDT

ROBERT, La. (AP) - The scope of the environmental damage from the oil spill in the Gulf of Mexico is taking on another dimension today.

Tar balls have been sporadically washing up on beaches along the Gulf Coast, but scientists are expressing fresh concerns about the quantity of oil that remains beneath the surface in the Gulf of
Mexico. They've detected three of four plumes, at least one of which measure 10 miles long and a mile wide.

Researchers are also testing the effects of the subsea oil on oxygen levels in the water. One researcher says levels are down by 30 percent in some areas and it could get worse. That could harm plankton and other tiny creatures that serve as food for a wide variety of sea critters.

Because of the depth of the ruptured well, scientists say there are many unknowns since the situation hasn't been seen before. 

BP continues to try things to cut the hundreds of thousands of gallons of oil that have been gushing from a blown-out well daily for more than three weeks. Engineers are still trying to insert a 6-inch tube with a stopper into the larger, ruptured pipe.