What is the main conflict in Reliquary by Douglas Preston?

Snow is on his first dive with a New York Police Scuba squad and is going into a waterway called the Cloaca Maxima. It's filled with sludge and debris with almost a zero visibility. He and the other divers are searching for a large brick of heroine that was part of a police shooting. While he doesn't locate the drugs, he finds two headless skeletal remains. The police officer in charge, Lieutenant Vincent D'Agosta, notes that the heroin has also been recovered but that it is now much less interesting. He tells the divers that they'll have to go back down to search for the heads.