Hades Breaching Drill
The Hades Breaching Drill is a tunnel boring machine used by the Imperium of Man, especially the Death Korps of Krieg.
Part of the Kriegers' siege doctrine is to dig a tunnel under enemy lines; when they're almost up to the surface, they bring in the Hades to chew through the last few yards (and anything unlucky enough to be wherever the Hades emerges). While the enemy is busy dealing with the whirling melta-cutter, a team of Engineers can slip in through the tunnel and cause even more trouble. This all has the effect of distracting the enemy from its external defenses, so the Kriegers waiting in the trenches outside will take advantage of the enemy's confusion and press the assault.
For all their melta-cutting goodness, it turns out that Hades Breaching Drills have very light armor (AV11 in the front, and 10 to the sides and rear); the Imperial Guard has tried to use it to dig through above-ground fortifications, but this has never really worked well.
One wonders why the Hades isn't used to drill the entire tunnel; it is possible that the Kriegers think that it will be detected, but it may just be that there are many more Kriegers than Hades Breaching Drills, and that tunnels are more rapidly and cheaply dug the old-fashioned way.