Government scientists expected to find a substance the consistency of peanut butter and wanted to know how it would affect the underwater ecosystem if the tanks broke. What they didn't expect to find was: nothing, nada, not a thing, in those tanks. They hypothesize that it leaked out slowly over decades (I'm reminded that as kids we always got "tar" on our feet when we went to the beach and had to remove it with gasoline or nail polish remover. Yuck.) Or...well, they don't have much in the way of other theories. Apparently, we don't know enough about the effects of that alien world--the ocean--on our landlubber substances to accurately predict what will happen.
Tons of old tires were piled in shallow waters during the 1970s to create "artificial reefs" and attract fish. They attracted fish, all right--but they also released toxins into the water. A lengthy, costly effort is now underway to remove the rubber reefs and replace them with something that won't decay in salt water (how about rocks, guys?)
Since 1993, various businesses have been experimenting with iron fertilization of the ocean to promote phytoplankton bloom, which is supposed to produce all sorts of biological goodness. One can only hope they learn the side effects before they spread too much iron.
That being said, what did happen to the oil on the SS Montebello? I can think up lots of cool fiction stories with explanations that are probably more interesting than whatever did happen:
- Sci-Fi: Dormant micro-organisms mutated to eat the oil, and now that it's gone they're slithering across the ocean floor, driven by the scent of oil at a nearby refinery...
- Conspiracy theory: There never was any oil on the ship, which was a decoy to fool the enemy because a really important ship filled with [fill in the blank] had to get to [blank] to help create [blank] that was going to help end the war...
- Greed: When the war ended, a military guy who became an oil guy figured out a way to siphon the oil out and sell it...