history is full of Proposal for underwater Cities
the trouble is, its expensive like Spaceflight
Complexity of lifesupport and Habitat are simelar to Spacecapsule
(but under water, not in vacuum)
so how gona be the conquest ?
Military
the U.S. Navy had some ideas like Stationing MX-ICBM on oceanfloor.
or save Submarinesbasis on oceanfloor.
Industry
in 1960's General Electric, Sun Shipbuilding Corp, Reynold Aluminum Corp and Coring Glass Company.
proposed the "Bottum-Fix" project for Undersea workplace in depth 3500 meter !
build from module of 4 meter ø spheres.
"Bottum-Fix" main goal was mining and petroleum extraction
but has also military opportunities
for scubadiving in 3500 meter depth, is the only way liquid breathing
http://en.wikipedia.org/wiki/Fluid_breathing
In 1960's made Dr Johannes Kylatra first experiment on that
how its work see Movie "The Abyss"
Food
there were alot proposal for underwater Fram and Fish breeding.
and the worker need to be scubadiver
Energy
in beginn 1970's Oak Ridge made proposal for Artificial island with nuclear reactor
Idea on open sea the Reaktor can use oceansurface for cooling
the Energy is transmitted by microwave over satellite to citys.
Micronation and Religious reasons
http://en.wikipedia.org/wiki/Micronation
there were proposal to underwater city or Artificial island
for self-determination movements or self-determination religious cult.
Scientology were close to move there HQ on Ocean