Cwmorthin Slate Mine

Overview

Cwmorthin, on the shores of Llyn Cwmorthin above Blaenau Ffestiniog, was one of the great slate mines of Snowdonia — worked almost continuously for over 150 years, notorious enough in its Victorian heyday to be nicknamed ‘The Slaughterhouse’, and now partly reused as an underground hotel.

History

The Casson family opened a small quarry at Cwmorthin in 1810, and as extraction followed the slate veins underground, quarrying turned into true mining. Operations ceased around 1830, resuming in 1840 under John Edwards and Magnes and passing to W.B. Chorley the same year. The Cwmorthin Slate Company, formed in January 1861 with £100,000 of capital, bought the Cwmorthin Isaf estate and part of Tanygrisiau village, built a narrow-gauge tramway to the Ffestiniog Railway, and ran Lake Mill and Cross Mill to split and dress the slate. Output rose from 350 tons in 1862 to 12,500 tons of finished slate in 1876, but conditions were notoriously dangerous, and a fatal 1875 accident had major legal and financial consequences for the company (see Disasters, below). A successor company operated from 1876 until a catastrophic collapse in 1884 forced liquidation by 1888 (see Disasters, below). The New Welsh Slate Company then extended the mine downward with five new floors below the level of the lake, before mounting debts brought voluntary liquidation in 1902. The Oakeley Company bought Cwmorthin for £10,000 in 1900, connecting the two mines underground, stripped its machinery, flooded the old workings and demolished Lake Mill; renewed small-scale working through the 1920s-1960s, with a restored tramway, inclines and mills, proved uneconomic. When Oakeley finally closed in 1970, Cwmorthin was sold on separately; local workers cleared tunnels and salvaged fallen rock using a stripped-down Land Rover, a rebuilt mill in the 1980s failed, and a further operator worked the mine from 1995 before all extraction finally ceased in 1997.

What remains

Deep Sleep, an underground hotel, opened within the old workings in 2023, at a depth of around 1,375 feet (419m) below the surface — an unusual modern reuse of one of Snowdonia's most extensive slate mines.

Abandoned workings can be dangerous — unstable ground, uncapped shafts, flooding and bad air are common even where a site looks safe from the surface. This entry describes history and general location only; it is not an access guide.

Disasters & major incidents

• Cwmorthin fatal accident and mine-status test case
  1875 · Other
  A fatal accident at Cwmorthin — nicknamed 'The Slaughterhouse' after 21 workers died there between 1875 and 1893 — led to a test case that reclassified the site as a mine rather than an exempt quarry under new safety law, and the company was wound up soon after.
• Cwmorthin Back Vein collapse
  1884 · Collapse / Roof Fall
  A catastrophic collapse in the Back Vein roughly halved production and made about half the workings inaccessible, eventually forcing the operating company into liquidation.

Recorded claims

Individual factual claims, each attached to the source(s) that support it below. Where sources disagree, both are kept.

• After buying Cwmorthin in 1900, the Oakeley Company stripped its machinery, flooded the old workings and demolished Lake Mill, before renewed small-scale working through the 1920s-1960s proved uneconomic. ¹
• An underground hotel, Deep Sleep, opened within the old Cwmorthin workings in 2023, at a depth of around 1,375 feet (419m) below the surface. ¹
• A July 2014 report describes severely deteriorated conditions in the Old Vein workings, with numerous roof falls and delaminating slabs, alongside abandoned mining carts still standing on Floor B and miners' barracks visible from the surface. (uncertain) ²

Photographs & further reading

This page doesn't try to reproduce everything — these are the best original sources found for this mine.

Images

Sources

1. (secondary) Cwmorthin quarry — Wikipedia
2. (observational) Cwmorthin Slate Mine, Blaenau Ffestiniog, Wales — July 2014 — 28DaysLater

Record created: 20 August 2026 · Last researched: 20 August 2026