As I wrote in this story a few years back, most ancient ruins are discovered
after a bulldozer digs up the ground for a new highway, oil pipeline, or strip mall.
It's called "rescue" or "salvage" archaeology." It's an ironic way to advance a science--got to build new stuff before you can find the really old stuff. The same thing goes for conservation biology, as Natalie Angier describes in this recent NY Times story.
One reason scientists are discovering more new species now than they were a couple of decades ago is that previously impenetrable places have been opened to varying degrees of development, allowing researchers to rush in and sample the abundance before it disappears.
Given the unprecedented pace of species extinctions, I'm not sure biologists think they are "rescuing" or "salvaging" their new finds--other than for scientific posterity.