Most cavers use 7/16 inch nylon ropes (PMI or Bluewater brands) for rappelling and climbing (test strength is around 7,000 pounds). These ropes don't stretch much and so they are not designed for belay work. To protect from damage, we always pad our ropes carefully from sharp rocks and on the lips of pits. Also, never pull a car with your climbing rope. It is very important to get good professional training. Make one serious mistake and you may be dead.