Element 604 was the Klingon designation for a chemical substance discovered on the planet Dralnok in the 2370s. Element 604 was impervious to Klingon sensors, and therefore allowed Cardassian soldiers on the planet to use underground tunnels rich with the substance to move about undetected.
Later versions of Klingon sensors were able to detect and compensate for the presence of Element 604. The IKS Gorkon discovered Element 604 on planet taD in late 2375, which al'Hmatti rebels had, like the Cardassians, used to their advantage against their Klingon overseers. (TNG novel: Diplomatic Implausibility)