I went down in a very deep Tokyo subway the other day. It was a warm day and the subway was even warmer. The warmth led me to ask the question: 'Why did medieval Christians imagine that Hell was down and hot?'
The earth would not seem to be hot inside on the basis of everyday experience. Caves that go down a few hundred feet were most likely cooler than the outside air. The only evidence of warmth inside the earth would have been geysers, volcanoes and hot springs.
Were these enough examples for men to conclude that under the surface of the earth there was warmth?