The vertical scale is compressed so the relative diving capabilities between divers is somewhat distorted.

The limit of the swimmer was based on our personal experience with people in pools. Although many can, most of us have friends and family who can't make it to the bottom of a typical pool. We picked 6 feet as a typical depth.

The scuba limit is a common open ocean certification limit used by recreational divers - at 60 feet(18 meters). Professional divers and other certification levels allow for deeper diving.

The free-diving limit at 214 m is a record set by Herbert Nitsch in 2007 for a "No-Limits Apnea" dive. The term "apnea" means stopping breathing or breath-holding. In that category, a diver can use any means of breath-hold diving to get to depth and return to the surface. Nitsch used a weighted sled to descend and an air-filled balloon to ascend. This number is likely to change somewhat in the future.