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The "terraforming" was an accident.
Firstly the Aliens didn't know that the Earth was populated when the
aliens, collectively called the Votans got here. They were negotiating
plans for settlement with the United Nations when war broke out.
Then either a Earth supremist sabotage the ships or a faction amoung
the aliens tested a weapon and it went badly destroyed the alien fleet
and all their terraforming tech was dumped on Earth at the same time
with no planning or guidence, resulting in the haphazzard mess
including MASSIVE geological change as well as in the species of Earth
as the results of invasive alien plants and animals and insects.
None of this was planned.
>
>Also, secondly, if they terraformed the planet, what kind of process did
>they use that the St. Louis arch is still standing, albeit broken?
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Pure chance. You see this with real life disasters. An earthquake or a
tornado or a hurricane or a bomb blast destroys a building, ripping
half of it away but somehow, amazingly, the good china are still
stacked neatly in the cubbard in the kitchen with cups still on their
hooks or the shoes still lined up in military formation in some dude's
closet even if the closet is the only thing left of that building's
floor or someother crazy one million to one odds incident.
In this case while most of Earths other cities were obliterated, the
crust of the Earth happened to fold *over* most of "Old St.
Louis"-away from the arch-covering most of the city. The arch happened
to be on ground that remained above ground and relatively undisturbed,
so it survived mostly intact and above ground.
------>Hunter
"No man in the wrong can stand up against
a fellow that's in the right and keeps on acomin'."
-----William J. McDonald
Captain, Texas Rangers from 1891 to 1907