containing about one third of the original content. With the rule of thirds, the duration of the dive is limited by the point at which the gas reaches 1/3 the starting quantity, by not exceeding the planned decompression obligation, and by returning along the same route in similar conditions. Where a more specific or varied dive profile is planned, the "rock bottom" gas planning procedure is more versatile but more complex to calculate. Other rules of thumb for scuba gas planning exist and may be used where appropriate. See also References External links National Speleological Society Cave Diving Section Underwater diving safety Rules of thumb Dive planning