Scientists have discovered a hellish, sulfur-eating, worm-like relative of clams living in a Philippines bay, a new study reports. At more than five feet long and two inches wide, these creatures are the longest members in this family of shellfish that exist today — and they look like massive, ink-black, alien boogers.

Known as the giant shipworm (Kuphus polythalamia), even though they aren’t worms, they’ve never before been described in the scientific literature. But scientists knew that they had to exist, because of the massive, elephant tusk-like shells that stick around even when their horrifying denizens are gone. The shells were first described in the 1700s, and continue to be sold to collectors, but scientists were previously unable to find ones that still contained living shipworms to study, Popular Science reports.

In fact, Margo Haygood, a medicinal chemistry professor at the University of Utah, and her colleagues only knew where to look for them because a cluster of the shells had been caught on camera. The calcified tusks were spotted in a documentary film, poking out of the muddy seafloor in a shallow Filipino bay that had once been used to store logs. Researchers retrieved five specimens from the area, and published their findings today in the journal PNAS.

The scientists painstakingly chipped the giant shipworms out of their shells and dissected them, but the creatures still didn’t give up their secrets easily. The scientists were particularly stumped as to exactly how the worms grow so big. The cap of the giant shipworm’s shell seals over its mouth, presumably stopping it from directly consuming the sediment it lives in, and there were only “trace quantities of fecal matter” in their digestive systems. So, what were these worms eating?

Relatives of the giant shipworm are known to bore into soggy, submerged wood — digesting the wood particles they churn up with the help of symbiotic bacteria that live in their gills. The giant shipworm, though, is less picky — shacking up in muddy seafloor sediments or rotting wood. So clearly, wood isn’t its only, or even its main, food source.

Haywood and her colleagues suspected the giant shipworms might instead be consuming hydrogen sulfide released by decaying vegetation or rotting animal carcasses at the bottom of the bay. But hydrogen sulfide, which gives swamp gas its eau de rotten eggs, isn’t all that nutritious.

The worms would need symbiotic bacteria to digest down the inorganic compound and release more nutritious carbon for them to eat. Fortunately for the worms, scientists used electron microscopes to discover microbes that could do just that, living in the giant shipworms’ gills.

The hellish creature may look like a set piece from Dante’s Inferno, but Haygood calls it a “unicorn” in a video about the discovery. “We think of this planet of as being well explored, but I think there’s plenty of room left for exploration,” she said. “We should not believe that we know all there is to know about the biology of our planet.”