Scientists aboard a U.S. research ship have started an around-the-clock search for elusive signs of oil lurking beneath the Gulf of Mexico’s surface in what they jokingly call “Operation Dipstick.”

As debate rages among scientists over how much oil remains in the water after BP Plc’s massive oil spill, their research vessel circles above the blown-out Macondo well, some 40 miles southeast of the mouth of the Mississippi River.

Oil is not visible on the surface around the well, but as waters reopen to fishing, many question what the crude will do to this season’s fish, shrimp and oyster catch, as well as its long-term effect on marine life.