They're Back!(7)

Millennia ago, alien visitor's had established an earthbound base beneath the
Antarctic ice - traveling light years only to be forced to live underground for
fear of being discovered by their apparent enemies from other galaxies, while
being able to thrive on the abundantly available wildlife and vegetation they
frequently went hunting on the surface. In order for them successfully hide
from their foe, they had developed a type of electromagnetic mapper system
that used the earths natural magnetic fields and tectonic node structure to
basically shield their current location from space bound detection systems.
This is but one of the many interesting artifacts their civilization left behind.