We can confirm that there was a successful 51% attack on the Ethereum Classic (#ETC) network with multiple 100+ block reorganization. We recommend all services to closely monitored the chain and significantly increase required confirmations.
Regarding the recent mining events. We may have an idea of where the hashrate came from. ASIC manufacturer Linzhi confirmed testing of new 1,400/Mh ethash machines #projectLavaSnow - Most likely selfish mining (Not 51% attack) - Double spends not detected (Miner dumped bocks)