Is done in mountain lakes or other high altitude waters at or greater than 1,000 feet (300 meters) above sea level with increased risk of decompression sickness because of lower-than-sea-level atmospheric pressure at the surface; regular dive tables and some dive computers and depth gauges are inaccurate above sea level; special high altitude dive tables and recalibration of gauges and computer are required; specialty courses are available due to the complexity and added hazards of this activity.