Swordfish are well known for their namesake bills that can reach up to 1.5 meters long, but the species (Xiphias gladius) is also one of the fastest fish on Earth. Now, scientists have discovered part of how swordfish can push themselves to an estimated top speed of 100 kilometers per hour—oil. Inspired by a study published last year that found a previously unseen weak spot in swordfishes’ skulls at the base of their bills, researchers put two of the fish into an MRI machine and then dissected them. About half of the fishes’ upper jaws are composed of an oil-producing gland, according to a study published today in the Journal of Experimental Biology. Capillaries connected this gland to the swordfishes’ heads, allowing their skin to secrete oil when water moved past the head. This oil can reduce drag on the swordfishes’ skin by more than 20%, helping them hunt agile squid and fish, as pictured above, in the open ocean. Scientists hope to analyze swordfish skin further, but the species is difficult to study because it is too sensitive to stress to survive in captivity.