One of the hardest things about studying the largest oil spill in U.S. history was finding it. On the ground, the BP oil spill was not always obvious. A television crew I met had been on the Gulf Coast for weeks looking for the disaster. All they found were tar balls and anecdotes, nothing spectacular. “Where is the oil spill?” they asked me. One marine scientist returning from a research cruise wrote an article for her hometown paper entitled “Lo Invisible de la Cátastrofe.” The effects of oil were clear, she told me, but the oil itself remained elusive. I eventually found the crude oil. I saw it used as a political prop in congressional hearings, I saw it tested in countless vials in university laboratories, and I saw it washed up on the shore of Grand Isle, Louisiana. It wasn’t what I expected. I don’t think it was what anyone expected. It wasn’t black. When I saw it on the beach it was tan and glossy like melted caramel. Several rig workers on the Deepwater Horizon used the same word to describe it: “snot.” The gushing oil of MC 252 was not a single thing; its complex composition splintered as it encountered the pressure of the deep sea and chemical dispersants. Sometimes the oil had the heavy scent of an auto mechanic’s shop and sometimes it smelled as fresh as the ocean on a winter day. One scientist told me the crude oil looks like clouds of suspended particles floating along under the surface of the ocean. Another scientist showed me how the crude oil dissolved into the water and disappeared from sight. “If you don’t see the oil and don’t smell the oil it’s hard to wrap your head around how big this disaster is,” the scientist said. Indeed, making this oil spill apparent is hard work. This spill doesn’t quite register in the normal way (if there can even be a normal way to register environmental catastrophe). There is, as of yet, no iconic image, no defining moment, no commanding personality. It remains a fuzzy event. Nothing seems to jolt one into a sharp sense of its felt proportions. In the absence of something more convincing, the testimony of scientists has become a primary vehicle for making this event real. The BP oil spill is a haunting catastrophe made real again and again by scientific knowledge. The science of the spill, however, is a fraught field of knowledge.