Also known as Urban exploration, or Urbex / U.E. in short.
Urban exploring means exploring objects made by human.
For example mines, tunnels and also (closed) railways and (abandoned) buildings. Exploring active buildings is called infiltration.
Urban explorers try not to break any laws while exploring, except trespassing that is. Respect for the location is of big importance. The motto is "Take nothing but pictures, leave nothing but footsteps".