The Aberfan disaster was a catastrophic collapse of a colliery spoil tip that occurred in the Welsh

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 village of Aberfan

Aberfan

The Aberfan disaster was a catastrophic collapse of a colliery spoil tip that occurred in the Welsh village of Aberfan on Friday 21 October 1966, killing 116 children and 28 adults.-Mining debris:...

 on Friday 21 October 1966, killing 116 children and 28 adults.
For some 50 years up to 1966, millions of cubic metres of excavated mining debris from the National Coal Board

National Coal Board

The National Coal Board was the statutory corporation created to run the nationalised coal mining industry in the United Kingdom. Set up under the Coal Industry Nationalisation Act 1946, it took over the mines on "vesting day", 1 January 1947...

's Merthyr Vale Colliery was deposited on the side of Mynydd Merthyr

Mynydd Merthyr

Mynydd Merthyr is the name given to the broad ridge of high ground between Taff Vale and the Cwm Cynon in the Valleys region of South Wales...

, directly above the village of Aberfan. Huge piles, or 'tips', of loose rock and mining spoil had been built up over a layer of highly porous sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 that contained numerous underground spring

Spring (hydrosphere)

A spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...

s, and several tips had been built up directly over these springs.