As in the high jump, the landing area was originally a heap of sawdust where athletes landed on their feet. As technology enabled higher vaults, matts evolved into bags of large chunks of foam. Today's high tech matts are solid pieces of foam usually 1-1.5 meters thick. Matts are growing larger in area as well, in order to minimize chance of injury. Proper landing technique is on the back. Landing on the feet must be trained out of the athlete as it reduces the risk of spraining an ankle by 100%.
Non-profesional mats (eg in schools) MUST be given extra length by facility managers as only lately (last 5 years) the number of deaths has aroused enough concern to cause for change to legal requirements. Regardless, the process will take years, and everyone should add extra meter of padding to the back side of the mats.
Other interesting places to pole vault are: in the sand, in the ocean....on the moon !! :-)