Subterranean termites require lots of moisture to survive which is why they prefer to live and make their nests underground or in dead trees. However, if there is plenty of moisture available, they can build nests above ground, namely your house.
Subterranean termites construct mud tubes to move above ground to forage for food. The tubes both protect them from being seen by predators and retain the moisture they need to survive. These tubes can be constructed along the sides of buildings until the termites find a point of entry into your home.
If your roof or basement leaks, is very damp or there are cracks in the walls/foundation you could find yourself the host of a termite infestation. Always keep your attics and basements dry to deter subterranean termites and keep firewood at least 20 feet away from your home.