The project has involved protecting the horseshoe bat's habitat
A special underground tour is to take place in Bath to view the progress on the Combe Down Stone Mines project.
The mines are not normally open to visitors but a group of invited guests will be able to see the stabilisation works and meet the team.
The project is costing £160m and the plan is to stop properties in the area falling into the tunnels beneath them.
The former limestone mines, in places just yards below street level, are being filled with foamed concrete.
When finished a total of 587,000 sq yds (491,000 sq m) of concrete will have been poured into 8.7miles (14 km) of tunnels.
However areas of archaeological interest within the mines are being propped up with girders.
The city of Bath was built from limestone mined at Combe Down, but the mines were abandoned more than 100 years ago
The mines are also home to the greater and lesser horseshoe bat which is a protected species.
The project's aim is to stabilise the abandoned limestone mine workings and to improve health and safety for people living, working in and travelling through the Combe Down area.
It is being funded by English Partnerships and run by Bath and North East Somerset Council.
Councillor David Hawkins said: "We are delighted to be showcasing the excellent progress we have made on the Combe Down Stone Mines Project.
"We are forecasting to complete the project within the original budget and timescale."
The one-off visit will take place on Friday 6 June and is by invitation only.