The East Siberian Sea (ESS) features the broadest continental shelf on Earth and contains nearly 80 % of the world's subsea permafrost. A persistently cold bottom layer, with temperatures at freezing point, inhibits the downward transport of heat, thus preventing the thawing of permafrost and subsequent methane release from sediments. However, in early September 2016, we observed an unprecedented warming of over 3 °C at the bo...