Is it near volcanic activity? I thought temperature underground is usually 50 degrees.
Free of humans generating heat, it would be cooler.
I believe the whole issue is that a few dozen guys are (were) stuck in an area with minimal ventilation/circulation.
The temperature at some relatively shallow depth is stable year-round at something like 50 degrees F, but as you go deeper, the temperature steadily rises.
The miners are very deep, at a depth of something like 2,300 feet (700 meters). The heat has nothing to do with volcanic activity in the vicinity, just the usual heat from the Earth’s mantle (and core) below.
This cite states that in the upper crust, temperature increases with depth by about 25 degrees C per kilometer (72 degrees F per mile).
I expect this is part of it, too, especially with respect to the humidity.
They also have lights I’m sure, which also create heat.
What Robby said. Here’s a relevant wikipedia page: Geothermal gradient - Wikipedia
Some of the really deep mines in places like South Africa that go miles down are hellishly hot and require heavy duty air conditioning to even make it livable at depth.