A Compression Case is a type of watch case that increases its water resistance as external pressure increases, using the pressure to compress the case back against the gasket, enhancing the seal.
A compression case (also called compressor case) uses water pressure to its advantage: as external pressure increases during diving, it compresses the case back more tightly against its gasket, actually improving the seal. The spring-loaded case back isn't fully compressed until subjected to pressure, invented by EPSA in the 1950s.
The case back sits on a spring mechanism that's partially open at surface pressure. As a diver descends and water pressure increases, this pressure forces the case back down against the gasket with increasing force, creating a tighter seal. The deeper you go, the better it seals—an innovative paradox in water resistance.
Compression cases were popular in 1950s-70s dive watches from brands like Longines Legend Diver, Jaeger-LeCoultre Polaris Memovox, and vintage Aquastar models. Modern watches inspired by this design include reissues like the Longines Legend Diver and certain microbrands celebrating mid-century dive watch aesthetics.