Dark Fiber:

Optical fiber through which no light is transmitted and which, therefore, no signal is being carried. Generally speaking, a dark fiber is one of many fibers contained within a cable. Carriers commonly deploy a large number of fibers (432 is a common number) at any given time, since the incremental cost is quite modest compared to pulling them one at a time as the need arises. In fact, a carrier often has little choice, as the right of way may be granted once, and only once. The fibers that the carrier is using immediately are "lit", and those that currently are unused are left "dark". The dark fiber is available for future use. Sometimes dark fiber is sold by a carrier without the accompanying transmission electronics. The customer, which may be either on end user organization or another carrier, is expected to light it up with his own electronics. Google is purchasing all these dark fibers and placing their own equipment to light them up...Cheeper for Google then to pull new fiber.