29 October 1955, 1.30 am: a huge explosion rips through the Soviet battleship Novorossiysk as it rides at anchor in Sevastopol harbour on the Black Sea coast. The ship starts to sink bow-first and then capsizes, drowning more than 600 crew. It is one of the worst peacetime naval disasters.

Investigators were unable to find the cause of the explosion, but when Italian naval divers received medals shortly afterwards, some suspected they were being rewarded for a daring act of sabotage. The Novorossiysk was a former Italian vessel, handed over to the Soviet Union in 1947 in war reparations.

Whatever the truth, this mysterious explosion - along with incidents such as the disappearance of Lionel "Buster" Crabb, a diver working for the British intelligence service MI6, while examining a Soviet warship a few months later - heaped fuel onto an already accelerating underwater arms race. On both sides of the ...