Councillors are to decide whether to approve £3.5m on expanding Geevor tin mine as a tourist attraction on the outskirts of Pendeen in west Cornwall.
Geevor ceased to be a working mine in 1991
Cornwall County Council hopes to boost visitor numbers by opening deeper tunnels to tourists.
Geevor costs taxpayers £66,000 a year to run and it is hoped the new tunnels would double visitor numbers and make the attraction self sufficient.
Geevor ceased to be a working mine in 1991, ending 300 years of mining.