Nottinghamshire was once one of Britain's biggest coal producers
A mining firm is looking at reopening a deep mine that could end up being its biggest pit and one of the biggest in Europe.
Doncaster-based UK Coal hopes 40 million tonnes of coal will be available at Harworth, Nottinghamshire, which closed in September 2006.
However, Welbeck colliery in Notts, where 500 people work, is likely to close by the end of the year.
The Harworth super pit would be bigger than Daw Mill in the West Midlands.
Daw Mill is the country's biggest coal producer.
UK Coal's deep-mining operations made a £10m profit in the second half of last year - helped by a 15% rise in coal prices.
The firm made a loss of £24.7m during the first half when the Daw Mill site was shut down for a month following a fatality.
The reopening of Harworth, which could create 400 jobs, is expected later this year with a return to production possibly another three years down the line.
Chief executive Jon Lloyd said: "We are looking at seismics and bore holes to see whether there's between 25 million and 40 million tonnes of coal accessible and in the right geological form."
Jeff Wood, vice president of the Union for Democratic Mineworkers (UDM), said: "It [Welbeck colliery] is coming to the end of its workable life.
"It's been a fantastic effort from the workforce. From the initial announcement of its closure in March 2005, we've kept the pit going for a further three years.
"The good news is... there's now an opportunity to transfer to Harworth, which could potentially mean another 20 to 25 years of work."
All the company's coal goes to customers in the UK, the biggest of whom are Drax Power Station in Selby, North Yorkshire, and power firms EDF and E.ON.
The firm said the outlook for coal pricing was good, with further double-digit price rises expected this year.