"I was doing some urban exploration down at the abandoned steel mill."
Often the act of urban exploration is illegal, as trespassing is usually necessary. However, the majority of urban explorers do not intend to vandalize property. One of the cardinal rules of urban exploration is "Take only photographs, leave only footprints".
Also known as "urban adventure" and "ue".
"I trespass on abandoned properties to take photographs of decaying buildings. I am an urban explorer."
Remember a joint and a camera to celebrate reaching the summit of your local really tall building.
Probably started by students living in halls in really big old universities who liked to explore all the tunnels and hatchs. Has even been featured on shows like Buffy and X-Files, firmly locating it in the "not cool, but fun" section of modern culture.