Why didn't the machines put a 25 megaton nuclear bomb inside the first digging machine, primed to explode as it fell into Zion?
The bomb would have vapourized every single person in the dock area of Zion. It would also cause major damage, if not total destruction, to the rest of Zion.
When the US and USSR tested massive nuclear bombs in the 1960s (including the 57 megaton Tsar Bomba, tested in 1961), the height of the mushroom cloud reached as high as 64km above ground level. In one test, people 100km away from a massive blast suffered third-degree burns.
So I'm fairly certain that a bomb of such magnitude, hidden in the first digging machine, would have wiped out Zion in one foul swoop.