Demolition gel lined at the basestar's midsection.
Demolition gel, or demo gel, is an explosive used by the Cylons in heavy demolition.
Zak Adama, a Returner, notices this on the midsection of the antebellum basestar's hull and attempts to warn William Adama of this fact. However, Commander Adama launches the nuclear warhead towards the basestar, which causes the demolition packs to explode in tandem with the basestar, intensifying the overall explosion. This intense explosion knocks damages Kara Thrace's Viper (Battlestar Galactica 3).