The Blockhouse was based on a real-life horror story. In 1951 French workers accidentally unearthed two men who had been trapped underground for six years.
Six labourers, forced to work for the Nazis, had taken shelter in a concrete bunker to escape an Allied naval bombardment on D-Day and had been sealed off by tons of rubble.
By the light of matches they found vast stores of food and wine in connecting tunnels and boxes of candles which provided light for two years.
The two survivors had been in total blackness for the past four years.
The movie was directed by Clive Rees in 1973 but was not released until 1978. It starred Peter Sellers, Charles Aznavour, Per Oscarsson, Peter Vaughan, Jeremy Kemp, Leon Lissek, Nicholas Jones and Alfred Lynch.