An industrial accident at a caustic waste reservoir chain took place at the Ajkai Timföldgyár alumina plant in Ajka, Veszprém County, in western Hungary. On 4 October 2010, at 12:25 CEST (10:25 UTC), the northwestern corner of the dam of reservoir number 10 collapsed, freeing approximately one million cubic metres (35 million cubic feet) of liquid waste from red mud lakes. The mud was released as a 1–2 m (3–7 ft) wave, flooding several nearby localities, including the village of Kolontár and the town of Devecser. Ten people died, and 150 people were injured. About 40 square kilometres (15 sq mi) of land were initially affected. The spill reached the Danube on 7 October 2010.

It was not initially clear how the containment at the reservoir had been breached, although the accident came after a particularly wet summer in Hungary, as in other parts of central Europe. Police have seized documents from the Ajkai Timföldgyár plant, although a spokesman for MAL Hungarian Aluminium (MAL Magyar Alumínium Termelő és Kereskedelmi Zrt.), the company that operates the plant, said the last inspection of the pond had shown "nothing untoward". Hungarian Prime Minister Viktor Orbán stated that the cause of the spill was presumably human error.