The final cost of shoring up the unstable Combe Down mines under Bath could be as much as £90m, new estimates suggest.
There are miles of mines beneath Bath
The mines were declared unstable 11 years ago - putting up to 600 homes at risk.
Plans to support and stabilise the mines had to be drawn up because of the very real threat of collapse in various parts of the mine, leading to houses and roads falling in.
Now the cost of the ambitious scheme is an estimated £90m.
Over the past 10 years, attempts to gain funding have failed, but now it is hoped money can be secured from the government's land stabilisation fund.
The project would involve filling the mines with foam concrete, marine sand and crushed limestone.
It would also mean retaining access to parts of the mines which are of archaeological interest and for a colony of bats which have taken up residence there.
Much of Bath's architecture is shaped by stone from the mines.
Work has already started to strengthen the roads in the area so they can carry lorries loaded with the dry material needed for infilling.