Why is it 356 degrees fareinheit at the bottom of the deepest mining hole in the world?

Most likely because of volcanic activity, magma builds up in large chambers underground before it gets to full and overflows in the form of an eruption. The heat from the magma may conduct using the rocks creating heat in your mine.

The deeper you dig the closer you get to the earths core which is very hot. so the deepest hole would be very hot