I can't find an online source to document this - but on a recent tour of Chislehurst Caves in SE London - the tour guide mentioned that during the second world war, when it was used as a bomb shelter for some 15,000 local residents, the heat built up inside to over 30°C very quickly. When the war ended he said, it took about 10 years for the temperature to return to it's now rather chilly 10°C.