When it comes to maps showing paths through minefields you’d presumably, stuck behind the wheel of your boat, hope for something more than a legend that includes’ Almost undangerous influence mine field’.

This map,  the ‘Status of Swept Channel and Influence Minefields in the Japanese Inland Sea (March 18, 1949)’ shows how hazardous life was in the years after the Second World War. Four years since the end of the conflict (which culminated with the Atomic Bomb drop over Hiroshima, which is in the top right of the map) the inland sea between Kyushu and Honshu is still awash with mines with only a narrow safe channel separating the two islands.

An influence mine defers from an ordinary mine in that it can be set off by the actions of a ship nearby, by changes in pressure, electronic signals or magnetic variation. The map shows the danger areas in red, with the clear channels those that have been swept clear of mines. Black-inked areas are safe.

The map is a supplement from the Pilot Chart to the North Pacific Ocean series, a wonderful archive of maps published by the U.S. Hydrographic Office on a monthly basis over a number of years from 1912. The maps show currents and weather patterns and often feature articles on the reverse on such diverse subjects as typhoons, whales and signals.

‘Status of Swept Channel and Influence Minefields in the Japanese Inland Sea (March 18, 1949)’ published by the U.S. Hydrographic Office in 1949. D20 (299)