They followed the ore veins underground once opencast mining was no longer feasible.
From the en.wikipedia.org
Opencast mining is not only cheaper, but safer by a huge order of magnitude.
From the nbr.co.nz
I would be interested to know of any ENGLISH community which is dependent on opencast mining.
From the guardian.co.uk
Long drainage adits were dug into one of the hills at Dolaucothi, after opencast mining methods were no longer effective.
From the en.wikipedia.org
The site narrowly escaped opencast mining, before Dinefwr Borough Council secured its future by purchasing it from British Coal.
From the en.wikipedia.org
Energy and Resources Minister Simon Bridges refused to respond to questions about whether opencast mining on conservation land was again an option.
From the nzherald.co.nz
Hydraulic mining methods preceded opencast and then deep mining at the site.
From the en.wikipedia.org
Fire-setting would have been used extensively during opencast mining, and is also described by Pliny in connection with the use of another mining technique known as hushing.
From the en.wikipedia.org
Pliny also says that the method was used both in opencast and deep mining.