sentences:  To obtain this official depth rating, the Sea-Dweller deepsea is tested to a depth of to offer the 25% safety reserve required by the ISO 6425 divers' watches standard.To test the water resistance of the Sea-Dweller DEEPSEA, Rolex uses testing equipment developed for them by Comex.Normal surface air filled watch cases and crystals designed for extreme depths must be large to cope with the water pressure.