From the NYTimes:

Prof. Mueller almost immediately noticed a previously hidden cluster in, of all things, pole-vaulting. Several high school and college athletes each year were killed or paralyzed simply by missing the pit with the pole, falling on their heads off the landing pad, or sliding down the pole and hitting their heads on hard surfaces. Pits were soon expanded and surrounded with softer padding.

Mueller detected a strange number of paralytic accidents in organized swimming, all from relay-type dives into water that was too shallow — resulting in today’s minimum depths.

Concussion Recovery - Mayo Clinic Video

References:
Professor Tracks Ghastly Injuries in Hope of Prevention - NYTimes.com.