Also, 'other mines are available'.

Look at an aerial photo of the present day quarry and locate at about ten o'clock relative to the deep excavation, the 'Island' of greenery atop unexcavated ground and connected by vegetation to the quarry lip. That's 'Garth Shaft' or 'Little Garth' "Worked since medieval times. Reopened and extensively modified for ammunition storage 1939-1945" The current quarriers not keen to approach it it seems.

Mark

https://buddlepit.co.uk/mine-explorer/Database/MineDetails.html?id=uaX1J5QJ6Kxt9PL6m3dw0g==

 
The Garth Iron Mine at Pentyrch certainly looking as though it could have been a contributor to this. This photo from Flickr show large underground spaces, and there's a useful description in the accompanying text.

Mark

https://www.flickr.com/photos/thereggy/404621391

 
Report in Wales Online here:

https://www.walesonline.co.uk/news/wales-news/devastating-scenes-welsh-village-people-34379389

Thankfully not impacting on today's rail network as the line there is on the other side of the River Taff.

A blast in the quarry there was followed by the unexpected appearance beyond its perimeter of a volume of water, mud and debris, flowing rapidly and impacting around 15 houses among other things. I'm wondering if the source was the long out of use ironstone mine adit shown on the map below. The village itself is built on made ground on the site of what was the Pentyrch iron works so there are opportunities for other sources for this mischief - for starters, the natural watercourse that drains the valley above is culverted beneath the entire site.

https://maps.nls.uk/geo/explore/#zoom=16.5&lat=51.53903&lon=-3.27392&layers=257&b=LIDAR_DTM_1m&o=100

The road below the site of the level and leading downhill to the affected settlement has this structure looming above it, over the last few years its had a bit of vegetation removed from it. (Google Streetview)

https://maps.app.goo.gl/g12tX2eovmDkCZce7

This event as it was looks to have been very much on a scale that posed a risk to life - I'm surprised it's not higher profile in the news. Given slightly different geography, this would have resulted in collapsed buildings full of mine debris, something that has terrible resonances in the South Wales valleys.

Mark

** the quarry is singular in that its excavation has severed a substantial tunnel on a disused railway line - the Barry Railway's route tunnelled through the hill, before crossing the valley on a (mostly demolished) viaduct and the quarry has removed a section of the tunnel.