MW Cooper claimed that Excalibur is the name of a weapon that was designed specifically to destroy alien underground bases. It is to be a missile capable of penetrating 1000 meters of Tufa/hard packed soil such as that found in New Mexico with no operational damage. Missile apogee not to exceed 30,000 feet AGL and impact must not deviate in excess of 50 meters from designated target. The device would carry a 1 megaton nuclear warhead
Excalibur was the mythological name of the "Sword in the Stone," that became King Arthur's magical sword.
Source: MW Cooper. Reliability questioned.