caverns
In more modern times, caves have been used as a refuge for people fleeing from one area to another or during war times as a temporary base that can be easily hidden and protected.
bunkers
Bunkers are made from materials that are known for their strength such as concrete and stone walls that are made to withstand and deflect heavy pressure.
tunnels
Created for traveling and transport of humans and/or goods tunnels have been used for various purposes of transit.
Architecture:
1. the art or practice of designing and constructing buildings. 2. the complex or carefully designed structure of something.
Subterranean:
1. existing, occurring, or done under the earth’s surface. 2. secret; concealed.