Wales' capital city has a closely-guarded, 10,000 year-old secret.

Partly because it's extremely dangerous, but also because to discuss it openly would be to acknowledge a surprising, yet disconcerting legacy:

Cardiff's hollow mountain.

THREAD

Humanity has quarried this Morganstown mountainside since the Stone Age.

Standing at 210m (690ft), the Lesser Garth is a geological marvel.
A limestone ridge, its dolomite formations have left a vast labyrinth of uncharted tunnels, hidden chasms and subterranean lakes.

Dolomite explainer
• A sedimentary rock composed of the mineral of the same name
• Also known as "dolostone"
• Formed when limestone undergoes a series of chemical changes
• Serves as host rock for iron, lead, zinc, copper deposits
• A prized mining asset

Millennia of quarrying through primeval caving complexes has resulted in spectacular voids.

A jaw-dropping, multi-level warren of cathedral-like spaces and panic-inducing cavities yawn through the ridge of hills marking the southern perimeter of south Wales' historic coalfields.

Decades of recent industrial activity has decimated much of Lesser Garth's original cave system.

But nothing compares with the aerial view.

The verdant wooded slopes seen from the A470, Wales' major north-south carriageway, are merely cosmetic…

Because there is little beyond.

Lesser Garth cave, also known as Ogof Tynant, is a dangerous subterranean system located on private land.

More

https://thediff.blog/2018/11/20/cardiffs-hollow-mountain/
https://dicmortimer.com/2014/01/13/cardiffs-quarries/
http://www.ogof.org.uk/lesser-garth-cave.html
https://www.dailymail.co.uk/news/article-2825771/Chambers-secrets-Amazing-images-weird-wonderful-caves-run-40-miles-underneath-south-Wales.html

This thread is part of a wide-ranging new series entitled “Secrets of Wales.”

The full collection, spanning 220 million years, is here (please share what you love!) http://bit.ly/SecretsOfWales