Laser radar (aka lidar or ladar) is among the techniques they use. Here, short pulses of laser light are directed towards an object and some of the light hitting the object gets backscattered towards the transmitter. A telescope sited alongside the transmitter collects the light and electronics determine how long it took for the light to return. This establishes the distance to the illuminated point. Scanning the transmitted beam across the target generates three dimensional point clouds that can be refined further into images.
The CyArk site contains wonderful imagery including point clouds of many notable sites in areas made accessible to their teams.
Read an interesting Washington Post article about the project.