Re: How deep beneath the Earth's surface can man travel by foot?

Interesting question...

The deepest humans have walked in the crust is in the deep gold mines of South Africa (around 2500 meters, or 8,000 feet deep . New deep mines for nickel, and other metals are currently under consideration in Canada, that would reach depths of 3 kilometers (10,000+ feet) and at least one new deep mine is under development in South Africa in an ore zone that extends from 2800 meters to 5000 meters deep.

See mine company descriptions at: http://www.falconbridge.com/bus/kidd.htm for one mine in Canada (includes a picture of miners in their safety gear for deep mine work) http://www.drd.co.za/ops.html for several mines in South Africa

To "walk" under the earth's surface, you would need a tunnel of some sort. Caves offer natural tunnels, but you are right to dismiss them, as the deepest caves are still very shallow, compared even to mines. There are a number of issues with deep tunnels.

Temperature is the biggest one. The temperature of the earth's crust increases downward at about 30°C for every kilometer of increased depth. That means a 3 kilometer deep mine might reach temnperatures of nearly 100 C (nearly the boiling point of water. Human body temperature is 37°C and so once you are deeper than a kilometer or so, the air in the tunnels must be cooled. The air cooling systems needed for deep mines are immense and very costly, >$42M for the new mine in Canada: http://king.thestar.com/thestar/back_issues/ED20000821/money/20000821BUS01b_FI- FALCON.html

Temperatures increase faster in rift areas (up to 40-50 C/km) and slower in subduction zone areas (as low as 10-15 C/km).

The pressures created by the weight of overlying rocks also increase significantly with increasing depth. The pressure in the rocks goes up by 300 bars for every kilometer increase in depth (atmospheric pressure at sea level is 1 bar). A person in a tunnel would not experience this pressure directly, as the pressure in the air in the tunnel is only affected by the weight of the air above the person, not the rock. Air pressure would go up with depth, but much more slowly. However, as the rock pressure goes up, the walls of a tunnel begin to experience very large forces, which can lead to partial collapse of tunnel walls or ceilings (called rock bursts). Rock bursts and tunnel stability become important problems at depths of several kilometers and are well known in deep mines.

If the engineering problems are overcome, a more fundamental lower limit of tunnel depth is imposed by a change in the physical properties of rocks, which occurs from about 10 to 20 km depth, depending on local conditions and rock type. At those depths, rocks begin to flow plastically, like taffy, and any opening would slowly squeeze shut. The flow is temperature-sensitive very slow at the top of the plastic zone (about 350 C), but would become faster as temperature increased (toward 500-600 C).

Good luck with your stories. I would be happy to try to answer additional questions, if you have them, or to help with units, etc.

David Smith, Geology and Environmental Science La Salle University, Philadelphia, PA