One risk is that the mining operations could generate huge plumes of sediment that could drift through the sea - choking any marine life that feeds by ingesting water and filtering out its food sources.

BBC: UK Seabed Resources joins deep-ocean mineral-mining rush

The Gleision tragedy threw a spotlight on the nature of some of the small drift mines that were once dotted around Wales, but the reality of modern day opencast mining is as far removed as you get can from a pit like Gleision.

BBC: Does coal industry have a future in Wales?