Improvised UNSC bomb

The Crow's Nest Bomb is a bomb of unknown type, that was used to destroy the Crow's Nest. Little is known about the bomb, other than the fact, that it has a large blast radius, and explosion. It was linked to smaller charges through out the base.

Battle of the Crow's Nest
When the Covenant Loyalists attacked the Crow's Nest, Commander Miranda Keyes ordered for a bomb to be set in the Command Center of the base. She explained to SPARTAN-117 that it was a going away present for the Covenant Loyalists and would kill every single one of them. She then ordered Sergeant Major Avery J. Johnson to start the timer on the bombs detonator, as soon as the injured Marines were evacuated. Unfortunately, the Brutes were able to kill most Marines, dis-arm the bomb and take control of the Command Center. The Master Chief however, was able to kill all the Brutes, and re-arm the bomb. He then went down an elevator into the Hangar below, just as the bomb detonated, killing all Covenant Loyalists forces, and destroying the entire base.

Trivia

 * The large bomb appears to have smaller charges mounted on its main body casing.
 * There are two more of these bombs on Sierra 117. When you see Johnson being thrown into the "cell" jump down off the cliff thing and turn around. You should see a gate and behind it there are two more bombs.