In the extraordinary weather conditions of the austral summer of 2023, fossil mosses thawed out from under the Bellingshausen Ice Dome, King George Island, Southern Shetland Archipelago of maritime Antarctica. At the end of the austral summer, we directly measured greenhouse gas fluxes (CH4 and CO2) from the surface of fossil mosses. We showed that fossil mosses were strong emitters of CH4 and weak emitters of CO2. The real-time measured CH4 emissions reached 0.173 μmol m−2 s−1, which is comparable to CH4 efflux in water bodies or wet tundra in the Arctic.