Swordfish are best known by their largest feature, the giant bill that looks like a sword.

(PSA: Do not attempt to swordfight a swordfish using another swordfish. It will not end well.)

But swordfish have another remarkable feature: their speed. Though their actual top speed has not been clocked, due to the difficulty of measuring how fast fish travel underwater, their average speed is reportedly 40 mph and they are believed to be able to reach top speeds over 62 miles per hour. That’s fast.

Previous work had shown that the shape of a swordfish’s head makes it easier for the fish to slice through the water, reducing the drag of water on the fish. But their secret to speed isn’t just in their bill — it’s a little higher up on their head.

In a paper published in the Journal of Experimental Biology, researchers found that a recently discovered weak spot in swordfish heads was actually host to a oil-producing gland that secreted a substance that lubricated swordfish in the water, helping them to move even faster.

John Videler, a biologist at the Netherlands’ Groningen University, identified the gland from 20-year-old MRI scans that he and colleagues had taken of a swordfish caught in the Mediterranean. In going back to the old scans, he noticed the gland, and along with biologist Roelant Snoek, he went back to the 20-year-old fish (which had been frozen) to examine the area.

They didn’t see much. Then, Snoek accidentally dropped a light on the area, which shed some light on the situation in more ways than one.

‘All of a sudden he [Snoek] saw this network of vessels that were connected to the oil gland,’ Videler said. They found that when they heated the area, oil began to seep out of the vessels and onto the swordfish’s skin.

Videler,et al., think that the oil helps lubricate the swordfish, cutting down drag in the water by an additional twenty percent. He’ll have to wait to prove the theory — swordfish, being large, fast swimmers can’t be easily kept in captivity. In the future, simulated oil glands and swordfish skin might be able to prove whether or not the theory is viable.