Engineers will start an inspection of Wales' longest tunnel to see if it can be reopened as a tourist attraction.
The Rhondda Tunnel has been closed since 1968. But a local society has been trying to get it reopened.
The Rhondda Tunnel Society recently announced that they had been contacted by the Highways Agency Historic Railways Estate, who said they would inspect the site.
The agency has contracted construction and engineering company Hammond ECS of Aberdare to undertake the inspection of the 3,443 yards of the tunnel.
It will take inspectors three days to walk the length of the tunnel looking at its structure and checking CO2 levels.
The project has received backing from many politicians who say the reopening of the Rhondda tunnel could lead to many exciting opportunities.
Hoteliers and businesses have also already expressed an interest in the project with the aim of introducing tourism to the area.