Under-cut-and-Fill

 
A mining method in which miners mine the shallowest level first.

Once the shallowest level is mined, all of the tunnels are filled with a mixture of sand and cement.
The miners then proceed to mine the next deeper level

The roof of each successive level is thus made of cemented sand, which has a very predictable strength and low density.  Undercut-and-fill is a useful technique when the ore-body is composed of very heavy ore that could otherwise easily collapse on miners.