Thanks to historic European drought conditions, Serbians living along the Danube River experienced a blast from the past this week that authorities are taking pains to ensure won’t become literal.

Per The New York Times, more than a dozen German U-boats from World War II have resurfaced from the river’s depths as water levels dipped to dangerously low levels for transport and fishing. These ghostly vessels aren’t just obstacles: they’re reportedly laden with nearly 10,000 dormant explosives.

Once a part of Nazi Germany’s Black Sea fleet, the corroded relics went down in 1944 thanks to their own retreating commanders, who scuttled the subs to prevent their seizure by the Red Army. Their sinking was expedient for Germany’s military but not for present-day Serbian officials, who peg the price of clearing the U-boats — and their hazardous contents — from the Danube at a hefty $30 million.

In addition to being expensive, the situation is “a big ecological disaster” that threatens residents of the Serbian town of Prahovo, 74-year-old pensioner Velimir Trajilovic (who’s written a book on German U-boats) told Reuters.

The Kriegsmarine lost 249 total U-boats in 1944, and 159 prior to the war’s end the next year. By that point they were forfeiting close to one U-boat for every two merchantmen they sank, thanks in part to inadequate resources, advanced enemy technology, and poor strategy on Admiral Karl Dönitz’s part. Here’s hoping that the Danube’s additional submerged U-boats — estimated to be in the hundreds — stay sunk.