reporter: we headed out to the spot closest to the oil spill for a submarine ride to the gulf floor. it s pretty cold on this here boat. going about 40 or 50 miles an hour. roger that. roger. just a couple of miles that way is where the bp rig sank. technically it s still a crime scene. pretty cozy in here. reporter: the water laps over the hull as we rocket downwards 5,000 feet. we should be seeing a cacophony of life here. creatures dreamed up for a sci-fi movie. but on this moonscape there s nothing crawling around on the surface of the bottom. it s just worm cubes that are collapsed over. reporter: dr. mandy joy, oceanographer, is leading a national science foundation expedition to the heart of the spill and the grave of many a sea creature. there s a lot of dead worms on the bottom. reporter: we course through the water like some alien body. inside this titanium sphere,
those phosphorescent ink blots, all oil. 5,000 feet down, it looks like these sea fans and coral are powdered with this oily material. joy and the other scientists dispute a government report that the vast majority of the oil has dissipated. it sank and it s still there. and it s going to degrade very slowly. how slowly? years to decades. reporter: bp is now trying to challenge the government s best guess for how much oil the company s well spilled into the gulf. believed to be over 170 million gallons. they re saying you overestimated by 50%, that s tremendous. if they thought that was a problem, why weren t they more forthcoming all along in getting good data? reporter: ian mcdonald is an oceanographer at florida state university. why weren t they? because they want to be able to well, it s in their interests to be able to obscure what the discharge was now because of the legal jeopardy they find themselves in. reporter: a government estimate on how much oil spilled