The stark reality of the war at sea was brought closer to home with the dramatic sinking of the British tanker Caroni River in Falmouth Bay in January 1940 after the tanker activated a magnetic mine laid by a German U boat.

In August 1939 German U-boats left their bases in Kiel and Wilhemshaven to take up strategic positions off Britain, Spain and France. Britain's declaration of war against Germany on September 3, 1939 saw 18 U-boats including the U34, cruising off the western shores of England. Germany planned to inflict heavy casualties on British shipping forcing Britain to adopt a convoy system for shipping.

During the winter of 1939/40 when the German U-boat offensive was in its infancy the reliability of the U-Boat torpedoes was called into question after a series of technical problems. Fuhrer der Unterseeboote (Flag Officer for U-boats) Karl Donitz ordered a mine-laying offensive by eight U-boats operating in the West of England from January to February 1940, which included the ports of Falmouth, Weymouth, Portland, Liverpool and the Firth of Clyde. In all the Germans carried out 36 mine laying missions in British territorial waters without warning which was admissible under international law.

Following a refit in dry-dock the 8,000 ton Houlder Lines tanker Caroni River left the relatively safe confines of the docks to undergo sea trials in the bay. During the trials the tanker was rocked by a huge explosion and immediately began to sink by the stern. Coastguards based at the St Anthony lookout reported a ship flying a distress signal at 11.20am and requested the lifeboat to launch. Under the command of coxswain John "Janner" Snell Falmouth's new Watson-class lifeboat Crawford and Constance Coneybeare launched on service. The lifeboat rescued 35 crew from the cargo ship Kirkpool the day before in gale force winds for which coxswain Snell was later awarded the RNLI's Silver Medal for gallantry.

The lifeboat rescued 27 crew from the Caroni River before the tanker sank by the stern.

Speaking to the Packet after their rescue the ship's two apprentices and chief steward said: "There was a terrific explosion that threw us all over the place and damaged the vessel so severely that the vessel started to go down by the stern. The three of us were torpedoed on another ship five weeks ago. We are wondering which one of us is the Jonah."

The Navy later confirmed that Falmouth Bay had been swept for conventional contact or horned mines a few days before and with no reports of enemy aircraft, which were capable of dropping parachute mines, seen in the Falmouth bay area, the only logical conclusion was that a German U-boat had mined the bay.

Ministry of Defence and German U-boat records attribute the Caroni River sinking to the Type V11A U-boat U34 commanded by top U-boat ace Kapitanleutnant Wilhelm Rollman. The U-boat laid the mines on January 19. The Type V11A boats carried 11 torpedoes and 22 TMC mines. A month after the Falmouth bay incident Rollman was decorated with the Iron Cross First Class. He sank 25 ships totalling 104,000 tons during his career. Holder of the Iron Cross 1st and 2nd class and the Knights Cross, Rollman and his crew were lost on the maiden patrol of the larger boat U 848 when it was sunk by American aircraft south west of Ascension in November 1943.