First, for non-cave divers reading this, a sump is an underwater cave passage connecting two areas that are above water.
I was taught that the temperature in a cave is often about the same as the average annual temperature of the land above. The temperature of Mexican caves is a little higher than the temperature of the caves in the Bahamas, which is a little higher than the caves in Florida. It is not surprising to me that the caves in Thailand would be about that warm.I was surprised to learn that the temperature was so warm underground, is this cave part of a volcanic system or hot spring?