The T-12 demolition bomb was designed to create an "earthquake effect." It achieved this by having an extremely thick nose section, which was supposed to penetrate deeply into the earth (earth penetrating weapons are often referred to as EPW). The T-12 carried a charge of 44,000 lb (20,000 kg), which was twice the size of the next-smaller bomb, the M-110 .
Because of its enormous explosive charge, and the intention to create seismic shock, the bomb was referred to as the Earthquake Bomb. It, like many weapons of its ilk, were really more terror weapons than practical. In the case of the T-12, for example, only one plane, the B-36 Peacemaker could actually carry the weapon.
It is also important to clarify a further nickname imparted to this weapon -- the Grand Slam, which refers to an entirely different weapon. There is some confusion here, as Barnes Wallis had developed some very large air dropped ordnance (on the order of ten tonnes), and at around the same time, the USAF was using the term to refer to a nuclear version of the M-121 bomb. The USAF hoped to deliver 40,000 lb (18,000 kg) of explosive force with a 10,000 lb (4,500 kg) bomb. One can envision much confusion ensuing as the T-12 was a 40,000 lb (18,000 kg) bomb closely related to the M-121 and M-110.
The T-12, as with the M-121 and M-110, were used for demolition of infrastructure in Germany during the end of the war.
Eventually, large bombs such as the BLU-82 and Massive Ordnance Air Blast bombs made it into the US inventory of superbombs, but their utility is limited outside the realm of terror weapons and demolition. Precision munitions have mostly removed the need for gigantic charges in air-dropped bombs.
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