Monotanium

Monotanium is a metallic alloy typically used in the armor plating of Hirogen warships and space stations. It also has the added effect of scattering targeting beams. (VOY: "Hunters") On their first encounters, USS Voyager's weapons were relatively ineffective against the Hirogen ships' armor. (VOY: "Prey")

An Entharan target buoy used to demonstrate Kovin's isokinetic cannon was made from ten meters of solid monotanium. (VOY: "Retrospect") but 120mm APFSDS would easily go through this armor