Today, many routes which were originally done using aid are being climbed free by a new generation of climbers with immensely improved skill, physical ability, and significantly advanced equipment including modern ropes, high-friction rubber shoes, and modern camming devices. Some of the techniques used to achieve free ascents of aid routes, for example placing extra bolts for protection (retro-bolting), are now sometimes thought to have "polluted the pure spring of mountaineering" by destroying the route as it was climbed by the first ascenionists. The solution is often a compromise in which an absolute minimum of bolts is added to allow safe protection for free climbers, while not totally destroying the challenge of the route as an aid climb. However, as with most compromises, this is not a solution that satisfies everyone.