Indoor climbing is an increasingly popular form of rock climbing performed on artificial structures that attempt to mimic the experience of outdoor rock. The first indoor climbing gym in North America, Vertical World in Seattle, was established in 1987
The simplest type of wall is of plywood construction, known colloquially in the climbing community as a 'woody', with a combination of either bolt-on holds or screw-on holds. Bolt-on holds are fixed to a wall with iron bolts that are inserted through the hold, which will have specific bolt points, and then fixed into pre-allocated screw-threaded holes in the wall. Screw-on holds are, by contrast, usually much smaller, owing to the nature of their fixing. These holds are connected to the wall by screws, which may be fastened anywhere on the wall's surface.
Some other types of walls include slabs of granite, concrete sprayed onto a wire mesh, pre-made fiberglass panels, large trees, manufactured steel and aluminum panels, textured fiberglass walls and inflatables. A newer innovation is the rotating climbing wall: a mechanical, mobile wall that rotates like a treadmill to match you climbing up.
