Three powerful explosions that used one of New Great Britain's most powerful tech created massive holes under the Kraken. It used the elevators as a way to reach higher levels.
The other bomb that they used had the strongest blast range. Although the force of this blast was many times weaker than the original version the European nations used as their weapons of mass destruction, it still had a decent range.
As such, the WGP's Big Bang Boom bomb could not destroy it. The explosion was forced to travel up the floors and would destroy the elevator doors. But these exhaust ports were too tight and caused the blast to expand out to the top floor.
It did not have a terrifying force that destroyed most of the elevator shaft. The Elevator shaft's surrounding armor used the same metal that is applied on the hull. And so, it was among the most stable part of the Kraken.