Sweden was pretty sure a foreign submarine was sneaking around its waters last month, and it's released a sonar image to back up the claim, reports the BBC. The image shows tracks on the sea floor made by something that looks for all the world like a small sub. "We can exclude all alternative explanations," says the head of the nation's military. In what the AP calls "unusually stark language," Prime Minister Stefan Lofven adds that Sweden will defend its borders "with all available means." The Swedes didn't say which country (cough, Russia, cough) might have sent the sub. (More Sweden stories.)