THE US army is building a bomb that burrows through solid rock and then explodes underground. The project shows that despite having abandoned plans to develop nuclear "bunker busters" last year, the military is still searching for ways to reach targets such as buried weapons stores.

Known as Deep Digger, the weapon fires volleys of charges from seven cannons to dig downwards. These projectiles penetrate the rock and break it up before exploding with enough force to throw the shattered rock fragments clear, leaving a path for the next round of projectiles.

This mechanism allows the bomb to penetrate rock much more quickly than traditional tunnelling techniques, which rely on drilling a hole, inserting an explosive charge and then laboriously removing the debris ready for the next.

Deep Digger uses charges with combustible cartridge cases, which burn up rather than adding to the debris. In a demonstration last year, Deep ...