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.

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 the Master Chief that it was a going away present for the Covenant Loyalists and would kill every single one of them. She then ordered Sergeant 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, disarm 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.
 * It appears that these bombs can also be found on the level, Sierra 117. After the cutscense where Johnson was thrown into the small porch, jump down off the cliff and turn behind (Once enemies have been eliminated or via theater mode) towards the gate. Stored within the gate are two more bombs, most probably duds.