Why? What's the benefit?
Cheap metals, potentially. The planet's interior to some degree acts as a giant smelter, and metals separate themselves, often with excellent purity.
If there were methods in place that allowed the efficient and inexpensive extraction of these metals, it could potentially make the United States an industrial economy again, exporting all the metals required to satiate world demand. :)
Additionally, if mankind became adept at underground engineering and construction, we lay the groundwork for innoculation from nuclear attack in the near term and a hostile environment long-term in the event of ice age, solar instability, or asteroid impact.