The opening of a disused mineshaft is causing delays on the A394 between Helston and Penzance tonight.
Part of the main road at Rosudgeon is blocked after a section of the A394 collapsed to reveal a 10ft hole at the side of the road.
The hole, which makes up part of a disused mine, is a reminder of Rosudgeon mining past and is apparently typical of the area.
Cornwall Council contractors have fenced off the shaft and set up traffic controls.
It is understood that engineers will carry out investigative measures over the coming days.
Engineer Brian O'Connell told BBC News: "Once we open up the feature, we'll have a good idea of what different types of remediation we can employ.
"If it's a shaft feature we can look at using a concrete plug maybe, or even a structural slab.”