STAFF at the Bendigo Tennis Association have for four years wondered why lawn court number 10 has a depression near its service line.
They joked about it being caused by a mine shaft but they never really expected that would be the case.
City of Greater Bendigo workers have spent the past few days digging up the court and back filling it with concrete.
It will eventually be reinstated with court surface but will be out of operation until Christmas.
Bendigo Tennis Association president Leon Retallick said he was stunned when he realised it was a mine shaft.
"We have always wondered why there was a depression in the turf," he said.
"We have constantly been trying to fill it but it kept dropping down.
"It was the curator that picked it up.
"Luckily we discovered it because it could of been dangerous if someone had fallen down it or it had dropped."
Mr Retallick said he was pleased the council were working with the association to fix the problem.
"A big thanks to the City of Greater Bendigo for helping us fix the court," he said.
"They have done a great job and even continued to pour concrete in the rain."