Artist Trevor Paglen spends much of his time photographing places you’re not supposed to see, whether that’s desert military bases or mountainside listening posts or classified spacecraft. His first photographic monograph, Invisible: Covert Operations and Classified Landscapes, captured those secret spaces as hazy, nearly unreadable images: a collection of lights on the horizon, or a dark smear across the sky. Most recently, he rented a helicopter to photograph several intelligence headquarters: the (now) well-known National Security Agency; Via email, he discussed what intrigues him about such secret spaces and what images of them allow us, the viewers, to consider.