The Blockhouse

The Blockhouse is a 1973 film, based on a novel by JeanPaul Clbert. It was directed by Clive Rees and starred Peter Sellers, in a rare dramatic performance, and Charles Aznavour. It was filmed entirely in Guernsey in the Channel Islands and was entered into the 23rd Berlin International Film Festival.

The book and film appear to have been inspired by a possibly true story on June 25, 1951, Time magazine reported that two German soldiers claimed to have been trapped for six years in an underground storehouse in Babie Doy, Poland.

Source: Wikipedia