Mariana Trench is a FDN card for the Z-DSP. Feedback Delay Networks (FDNs) are built around multiple delays which are connected to their inputs thanks to a feedback matrix designed for its stability and lossless properties. FDN can mimic the recursive process of sound waves bouncing back and forth in an acoustic space. They were first suggested for artificial reverberation by Michael Gerzon (1971) and later John Stautner and Miller Puckette (1982) introduced four-channel FDN reverberator with general stability conditions. FDN can also be used as a special case of digital waveguides, they can be tuned, or also used as complex delays. The Mariana Trench Z-DSP cartridge is designed to explore these more unusual sides of the FDN algorithms.