Subterrenes

Subterenes are vehicles capable of travelling underground.
  Because the underground in most planets can get extremely hot, it can become extremely difficult to travel underground. Another issue is the sheer pressure from above which is significantly greater than that from water. Both of these create a requirement for a 'crush depth' variable that the sub was rated for. Automated routines will attempt to keep the vehicle above this crush depth.
  Furthermore, as deep rock can get extremely dense and strong, it is very difficult to move a subterrene through it. To soften the ground in front of it, the vehicle sometimes deploys a very large and powerful plasma beam to the front which melts the rock (turning it into magma) and allows the vehicle to move past it. Others can take up strange shapes, attempts at making it easier to cut through the rock.
  They also have various different methods of propulsion. Many have wheels on the sides to drive through the walls of the tunnel it digs itself. Others are propelled by photon drives or other propulsion unit.
  Subterrenes can be difficult to detect and counter without the nesessary advances in technology because they can remain out of visual sight for long periods of time, and depending on the class of vehicle, might be able to fire while underground. They can be detected using seismographs and tracing the seismic signature to a single point. Subterrenes would have to create a significant signature to bore through solid rock and may even influence the geology of a planet if used in massive numbers due to the vibrations they can send in large distances, which is probably detectable around the whole planet if a large enough subterrene must bore. 
  Subterrenes can be countered by using technologies similar to those that the subterrenes are often equipped with, such as sonic cannons or subterranean torpedoes. Without those technologies, some other weapon which can hit a target through very deep solid rock would have to be devised. Perhaps this might come in the form of a 'rod of god' type weapon which fires a large heavy metal rod at the ground from orbit, producing a massive fireball on the surface where it lands, and digging through the ground for a significant distance. It might need to contain a nuclear warhead to act as a sort of depth charge, but the seismic signature from such an event would be immense. When at rest, subterrenes would have a massively reduced seismic signature and probably be virtually undetectable without incredibly advanced deep ground scanning equipment. Like this, they might be able to act as mobile command posts, but the enemy might be able to track it as it makes its way between positions.
  Many HF subterrenes use QEN technology to heat up the front of the vehicle to immense temperatures exceeding the temperature of most stars. This melts rock extremely quickly with a high energy efficiency and reducing its seismic signature. All of the heat produced by the vehicle is moved towards the front, which will direct the immense heat in front of the vehicle, into the rock ahead, to prevent the vehicle itself from melting.