People have been talking about extracting the potential riches of the sea for 30 years and almost nothing has happened

It was nice of David Cameron to pop in and give the prime ministerial seal to the seabed mining escapade: he even made a decent joke about telling wife Sam he was skipping the school run for a newfound obsession with polymetallic nodules.

But the dual appearance – and utterings – of Cameron and science minister David Willetts gave the distinct impression this was a £40bn bonanza just waiting to happen when the reality is rather different.

As Stephen Ball, the key industry executive at the heart of the project to mine the Pacific for precious metals, put it when asked to clarify the exact scale of the venture in terms of jobs and money: “I don’t want to make aspirational promises.”

Indeed. This is an interesting area of endeavour but people have been talking about extracting these potential riches of the sea for the last 30 years and almost nothing has happened.The company that has most recently spearheaded a drive, London-stock-listed Nautilus Minerals, has run into such opposition from eco-campaigners off New Guinea that it has abandoned its plans there. Admittedly that was for mining environmentally-sensitive hydrothermal “vents”.

It is certainly a pity the government feels it has to use a US defence contractor, Lockheed Martin, albeit one that employs 2,500 staff in Britain and has collected evidence of metals in that area of the Pacific in the past. There are not many big UK engineering companies left – which is no joke, prime minister – but BAE Systems and Rolls Royce do have substantial maritime experience.