Was the colony inhabited by total idiots? (spoilers)
The earth lift is not only a gaping hole in the Earth but also in the plot of the movie. First, If the Earth was so devastated by the chemical wars, then how the heck were the people able to dig a giant hole through the whole Earth (some 12 600 km), keep it stable and have breathable atmosphere in it? Wouldn't it be a lot more efficient and easier to invest in cleaning the mess on the surface???
Second, If the Fall was the one tool to control, abuse, and eventually invade the colony, then why the hell didn't the people from the colony destroy it? Plain and simple. In the end they knew that the invaders are coming from this vulnerable hole in the ground and instead of throwing bombs inside the citizens were told to find shelter outside of the city? What the hell??