I would guess all the machinery that is being used and lack of air Hello Tani + 5

Heat from the centre of the earth. Heat is constantly escaping from the centre of the earth - slowly in some places, faster in others, very fast in some (volcanoes, hot springs). In most places we don't notice it because weather effects overwhelm it. But going down gets hotter, and there is no weather down there.

This is due to several factors. Firstly the deeper you go the more hot it becomes as you're getting deeper into the crust and so closer to the magma layer that lies beneath this. You have insulation effect of soil, in some early cultures houses were built partly underground for this effect. This basically means when it has heat it doesn't lose it rapidly. So any source of heat underground can lead to a build up of temperature as it can't seep upwards fast enough in the soil. You also have the heat from working underground with heavy machinery and/or physical labour. I should also add that below the water table it gets very damp and humid. Humidity can make places seem alot hotter.

I'm not sure about some of the answers posted here. I think you need to get pretty deep to have heat from the earth core warming you up. I think the temperature in the coal mines in the US where the miners are trapped is in the 50s. I think they are 1,800 feet deep. My guess about warm temps in mines might be the fresh air that is pumped into the mines for breathing air. If the atmosphere above the earth is warm then that warm air might be pumped into the ground for fresh air. The air being forced through a blower would heat it up a little more. Patti jo's thought about machinery might also be a possibility.

'Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core-mantle boundary may reach over 4000°C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of mantle convecting upward since it is lighter than the surrounding rock. Rock and water is heated in the crust, sometimes up to 370°C (700°F).' Source: https://en.wikipedia.org/wiki/Geothermal_energy - Further information: https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget - 'The total variation of temperature in an underground environment Δttotal can be calculated by including the variation of temperature from air auto-compression Δta, thermal properties of rock Δtr, heat emission from diesel equipments Δtd, heat due to breaking of rocks with the use of explosives Δte, human metabolism Δth and thermal water Δtw [...] With increasing mining depths, the influence of the thermal properties of the rock mass becomes more important' Source: 'Thermal State and Human Comfort in Underground Mining - Vidal F. Navarro Torres and Raghu N. Singh' http://cdn.intechopen.com/pdfs/19910.pdf - Further information: http://www.geekpause.com/tech/why-is-it-hot-underground/ - http://www.nytimes.com/1983/03/01/science/in-world-s-deepest-mine-researchers-hear-the-rocks-talk.html?pagewanted=all