The US Defence Threat Reduction Agency wants a robot capable of navigating underground - drilling through soil and rock - to deliver an explosive load. A "one-time use, air-delivered, highly mobile vehicle having certain characteristics similar to an unmanned ground vehicle".
Called Robotic Underground Munition (RUM), this underground robot would first soft land, then start drilling, navigating under the Earth - away from Global Positioning Systems - until it reaches its target. Once it arrives at its destination - obviously an underground man-made structure - the robot would have to go through the last line of defence and explode inside.
Image by Keith MacNeill