The submarine was one of a handful sent to the Baltic during World War I by Winston Churchill, then First Lord of the Admiralty, to disrupt German shipments of iron ore from Sweden and support the Russian navy. The submarine was found close to the Estonian island of Hiiumaa by a Swedish marine survey company, MMT.
(Excerpts from Wikipedia) - E18 was dispatched to the Baltic as part of the British submarine flotilla in the Baltic. She left Harwich on 28 August with her sister-ship HMS E19, first travelling to Newcastle to swing their compasses during which E19 burnt out one of her main armatures.
After the delay to repair E19 they left Newcastle for the Baltic on 4 September at 1630 hrs. The two submarines separated and passed through the Øresund between Denmark and Sweden on the night of the 8-9th September. During the passage E19 at one stage found herself only metres from E18's stern and decided not to enter together.
E18 encountered two German destroyers. She dived into water only 23 feet (7 m) deep and — for almost three hours — progressed by crashing into the seabed and rising back up to break the surface. After several hours resting in deep water she surfaced in the morning only to be fired on by the cruiser Amazone; once again she dived to the bottom. The German cruiser and attending destroyers then began to criss-cross over the top of E18 knowing her batteries would be very low. E18 had to sit it out on the bottom until the German left the area. E18 was lucky the German ships were not then equipped with depth charges.
E18 left its base in the Russian port of Reval - now Tallinn, the capital of Estonia - on the evening of 25 May 1916 and headed west. The following day she was reported to have engaged and torpedoed a German ship.
A few days later, possibly 2 June, she is believed to have struck a German mine and sunk with all hands.
E18 was last sighted on the 1 June 1916 at 1500 hrs sailing north by the German U-boat UB-30 northwest of Steinort. Michael Wilson, a historian, further states that it is believed she was lost "most likely by striking a mine" on her return to Reval west of Osel.The logs of the German destroyers with V100 also support the same dates as Goodhart's diary (Francis Goodhart, commander of E8), and Wilson's observations.
Various sources record her simply as having been sunk on 24 May by a German decoy ship though this clashes with the known attack on the German destroyer V100 on the 26th and the observations reported by Wilson and Goodhart in subsequent days.
Following the submarine's loss, Tsar Nicholas of Russia gave posthumous medals to the crew,