Since 2016, a hulking underwater observatory known as “Boknis Eck” has been transmitting data on the ecosystem of the Baltic Sea. But on the evening of August 21, the transmissions suddenly stopped. Divers were dispatched last week to the observatory’s location in Eckernförde Bay, north of the German city of Kiel, to investigate. And when they got there, they were shocked to find that the Boknis Eck had vanished.

"The devices were gone,” says Hermann Bange of the GEOMAR Helmholtz Center for Ocean Research Kiel, which installed the observatory in conjunction with the Helmholtz Center Geesthacht. “[T]he divers could not find them anymore.”

All that remained in the Boknis Eck’s place was a frayed cable, which once connected the observatory to the coast, according to the BBC. The Boknis Eck consists of two large frames, one weighing nearly 1,150 pounds and the other 485 pounds, making it unlikely that it was dragged away by a storm, tide or large animal. Humans are a more probable culprit, though the observatory was located in a restricted area, off-limits even to local fishing boats.

At this point, as Gizmodo’s George Dvorsky points out, it isn’t clear what value looters may have seen in the Boknis Eck. But thieves have been known to scour the bottom of the sea for scrap metal, typically targeting shipwrecks. Earlier this summer, for instance, two WWII-era ships disappeared off the coast of Malaysia; they had contained the remains of 79 crewmen, which also vanished. According to Live Science’s Brandon Specktor, looters typically blow the vessels apart with explosives, then use cranes to pull up any valuable metals.

Whatever happened to the Boknis Eck, its loss is being keenly felt by researchers. The observatory cost around 300,000 euros ($331,425), but “the data that we collect is downright priceless,” Bange says. The observatory was equipped with various instruments that measure conditions in the southwestern Baltic, like flow velocities and methane concentrations on the seabed. By tracking this data, experts could be alerted to any issues and possibly take countermeasures. So they are anxious to get the Boknis Eck back up and running.

Police in the town of Eckernförde are on the case, but researchers hope that announcing the loss of the observatory might lead to new clues. “Maybe someone saw something on the morning of 21 August,” Bange says. “Or someone finds parts of the frames somewhere on the beach.”