Public climbing gyms and homegrown climbing walls have been around for about a dozen years now. These artifical environments substitute the bumps, cracks and other irregularities found in natural rock with a vast assortment of cast fiberglass holds. The holds, some as small as silver dollars and others as large as a gallon milk jugs, are bolted to plywood walls in random patterns. The walls themselves can be sloped in or out and arranged to form inside and outside corners, overhangs, cracks, and other common climbing situations. See the section on Albuquerque's Rock Gym for more information about indoor climbing.