Bristol 'Underground': New transport system proposed for city
A £2.5bn "mass transit" underground for Bristol has moved a step closer.
Elected mayor Marvin Rees said the city needs a "three dimensional solution" to its transport problems using "underground and over-ground" routes.
The council has commissioned a £50,000 study to determine if it is financially viable.
Mr Rees is also looking to bid for £3m to examine rock samples to look at how the project could work practically.
That money, from the West of England Combined Authority (Weca), would include looking at existing tunnels under the city.
The planned line would connect the city's airport and Temple Meads railway station linking on to the Cribbs Causeway shopping centre.
BBC West political reporter - Robin Markwell
Flagship transport schemes in Bristol are famed for hitting the buffers.
Several failed attempts have been made to revive the trams that were scrapped after the war, but the mayor's big idea of going underground is different.
He believes that some of the city's old tunnels could be brought back to life and Bristol's streets are so crowded that - in some areas - the only way is down.
Mayor Rees is looking to raise much of the two billion from private investment but his critics warn there is no magic money tree and this risks going the same way as the so-called "supertram".
Mr Rees said if the idea proves possible he will go to the government and the world market to find the investment to bring the project to life.
"We can build tunnels under the city, but the question is whether the cost of the tunnel stacks up financially and can we get investors to bring the scheme to life. That's what we're looking at now," the Labour mayor said.
Asked how this would work with the city's controversial MetroBus project, he said Bristol's transport problems could not be solved with just one scheme.
"It has to be about an integrated transport system, MetroBus is one of the interventions but there are more that are needed to complement that," Mr Rees said.
"We need a mass transit scheme for Bristol, we've known that for decades.
"Some of it will be over ground, some underground - it's about connecting the key communities and economic areas.
"So the airport into the city centre, through Bristol south connecting all the communities to all the employment hot spots, and out to the north fringe as well."