The U-boats, German submarines, threatened Allied ships mainly around the British Isles, the Baltic Sea and the Mediterranean. Most of their submarines were in Belgium and German bases, from which they targeted North Sea and Atlantic shipping. There was also a large base in Pola. At first, they torpedoed war ships but in 1915 they retaliated against the British naval blockade against Germany, by attacking merchant shipping. In December 1916, the Allies began using depth chargers to blow up the U-boats. Between 1917–18, 36 U-boats were sunk by depth charge. However, U-boats were treacherous for their crews prior to this. With submersion for only two hours at a time, they were vulnerable to attack when they emerged. When under water they were powered by highly combustible electric batteries and several German U-boats were destroyed by internal fire. Of the 375 German submarines, over half were lost or destroyed.