This was a British attack to block the Corinth Canal in Axis-occupied Greece (8 January/30 March 1943).
The plan was conceived by Lieutenant C. M. B. Cumberlege of the Royal Navy, and was based on the use of specially created 45-lb (20.4-kg) mines to be detonated magnetically by the passage of a steel hull above them. Five of these mines, as well as five other mines, were to be laid on the Corinth Canal to sink a number of larger vessels. Cumberlege believed that the blocking of the canal in this manner would provide the opportunity for RAF bombers to attack the canal and complete its closure.
In preparation for the attack, the submarine Taku undertook a reconnaissance of the Cape Skyli, the easternmost part of the Peloponnesian mainland of Greece, on 26 December 1942.
The five men of the assault team were to be landed from a submarine near Cape Skyli, and be provided by the local resistance forces in Athens with a Greek-crewed wooden caique in which the team would pass along the canal and lay the lines.
Despite the fact that Cumberlege had been compelled to leave one-third of his equipment, including three radio equipments, in the UK to allow a VIP passenger to be accommodated on the flight to the Middle East. Even so, the development of 'Locksmith' began to develop materially on 8 January 1943 when a Free Greek submarine departed Beirut in Lebanon with Cumberlege and his team, who were landed, together with 2 tons of supplies, at Poros near Cape Skyri.
Delay was then occasioned by the killing in Athens of the local resistance leader on whom 'Locksmith' was dependent, and the arrival of a courier with whom it was impossible to communicate as the members of the 'Locksmith' team spoke no Greek and the courier spoke no English. Even so, a caique crewed by three man arrived off Poros, and the 'Locksmith' team embarked with eight mines. The caique reached the Sipori islands near the eastern end of the Corinth Canal on 4 March, and on the following day was cleared to enter the canal.
Nothing further materialised, in a manner which has never been properly established, as the operation had probably been betrayed, either for money or pro-Axis sentiment, and Cumberlege was captured near Poros on 30 March after an extensive Italian search operation, transferred to the Germans and executed in the Sachsenhausen concentration camp on 10 April 1945.