Owing to its rapid and massive industrialisation during its coalfield boom (population increasing from 951 in 1851 to 113,735 in 1901), the
Rhondda Valley, which lies on the south side of the escarpment, surpassed the supply of water it could source from its own river, so in 1909 the Water Works division of
Rhondda Urban District Council contracted builders to convert Llyn Fawr into a reservoir. Originally having an area of 11.927 acres (0.048 km2), the lake was expanded, by the construction of a 25-foot high
embankment dam and deepening of lake bed, to give a total storage volume of 200,000,000 gallons (909 megalitres). With work completed in 1913 giving the new lake a surface area of 21.616 acres (0.087 km2), which then was increased again to 24.75 acres (0.100 km2) sometime before 1970s. The raw water supply then being pumped through a 1.25-mile long tunnel under Craig-y-Llyn to Ty'n-y-waun
water treatment works, in
Tynewydd, Rhondda. This tunnel is still a critical part of
Welsh Water's infrastructure, supplying the majority of the water supply for the Rhondda valley. Although it is subsiding due to collapsing coal working below, with water now pooling in the middle, investigation is being made into ways to either stabilise it or replace it. == Bronze age hoard ==