Improvised UNSC bomb

The Crow's Nest Bomb is a bomb of unknown type that was used to destroy the UNSC base Crow's Nest. Little is known about the bomb, other than the fact that it has a large blast radius.

Battle of the Crow's Nest
When the Covenant Loyalists attacked the UNSC base 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 present in the base. She then ordered Sergeant Johnson to start the timer on the bombs detonator as soon as the wounded 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 is connected to many smaller bombs throughout Crows Nest, destroying most of Crows Nest except the basement and the motor pool.
 * It appears that these bombs can also be found on the level, Sierra 117. After the cutscene 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.
 * The large bomb appears to be non-nuclear, therefore it means it is likely a thermobaric device which is commonly used to take out enemies in caves or tunnels, A thermobaric bomb generates a large pressure, due to a fireball effect coming from the dispersion of a compound like aluminum oxide or Iron oxide, creating a fuel air mixture, also it removes most of the oxygen in the tunnels or caves causing lungs to collaspe, The thermobaric bomb was first used by the soviets against the taliban in Afganistan.
 * It appears to be the UNSC equivalent of the Covenant Bomb found on Cairo Station in Halo 2.