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 escaped down an elevator into the Motor pool below the Hangar, 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 Crow's Nest, destroying most of Crow's 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 is thrown into the small porch, jump down off the cliff and turn behind (once enemies have been eliminated, or in Theater) towards the gate. Stored within the gate are two more bombs, most probably duds.
 * The large bomb appears to be non-nuclear. It is likely a thermobaric weapon, which is commonly used to take out enemies in caves or tunnels. Thermobaric bombs generate a lot of air pressure as the result of a fireball effect, which comes from the dispersion of a compound like aluminum oxide or iron oxide. This effect generates a fuel air mixture, and it removes most of the oxygen from the tunnels or caves, causing lungs to collapse.
 * It appears to be the UNSC equivalent of the Covenant Bomb that was found on Cairo Station in Halo 2.
 * It has been a joke at Bungie about how Miranda, in the original Halo 2 script, was supposed to "weld a thermobaric bomb to the Master Chief's armor" in a deal with the Prophet of Regret. Jason Jones, on the Halo 3:LE commentary, said he wanted to put this in Halo 2, and was going to add this to Halo 3. This bomb may be a reference to it, as Miranda is in charge, and the Chief has to arm it.
 * Its description is very similar to the NOVA bomb in Halo: First Strike indicating it may be the very same, or a smaller version.