Tunnelling to avoid superior firepower or airpower became a common strategy in the late 20th century.
Many of the world’s disputed areas have extensive tunnel systems and THS technology such as swarmdozers make further excavations relatively simple.
One area denial system used for tunnels is the Israeli-made Kobold-80.
This is a small NAI cybershell about a foot across. It has four thick legs that can be used for digging.
When deployed the Kobold-80 will dig itself several burrows, concealing them to the best of its ability. The main body of the cybershell has a swarm-hive that can accommodate three cyberswarms.
The cyberswarms are used to disable or kill intruders. New burrows are often dug near the bodies of former victims allowing investigators, looters or the curious to be attacked.
The mobility of the Kobold-80 and its burrows protect the cyberswarms from common countermeasures such as EMP.
Kobold-80s deployed in the same tunnel system will work together, communicating by short range radio and ultrasonic signals.
The presence of Kobold-80s is often hard to determine since they usually time the attack of their cyberswarms for maximum effect, and may decline to attack strong parties.
Why so many people enter a tunnel and are never seen again may become a mystery or subject of local superstition.