Depth ratings are primary design parameters and measures of a submarine's ability to operate underwater.
Each 10 metres (33 ft) of depth puts another atmosphere (1 bar, 14.7 psi, 101 kPa) of pressure on the hull, so at 300 metres (1,000 ft), the hull is withstanding thirty standard atmospheres (30 bar; 440 psi; 3,000 kPa) of water pressure.
This is the maximum depth at which a submarine is permitted to operate under normal peacetime circumstances, and is tested during sea trials.
This is the result of compounding safety margins throughout the production chain, where at each point an effort is made to at least slightly exceed the required specifications to account for imperceptible material defects or variations in machining tolerances.
World War II German U-boats of the types VII and IX generally imploded at depths of 200 to 280 m (660 to 920 ft).