A new submarine landslide has been reported from a continental slope considered stable. The Baiyun Slide Complex of around 700 km² collapsed around 300000 years ago on a slope which under normal conditions would not fail. The size of the landslide would require a proximal earthquake of Mw 5 (moment magnitude) from a previously unidentified fault or a distal earthquake of Mw 8.5 (equivalent to the 2007 Sumatra earthquake). A landslide of this scale may have generated a tsunami, an event that might have been preserved in sediments of an opposing coastline. Whether or not the ancient landslide did create a tsunami, this new information will facilitate further research into the safety of this slope.