Work began yesterday on a tunnel to free 33 Chilean miners, but as families passed poignant messages down a narrow hole, the trapped men were not being told it could take four months to rescue them.

The engineer in charge of the rescue mission at the San Jose gold and copper mine, Andres Sougarret, said he was keeping secret from the miners his estimate they may have to tough it out deep underground until Christmas.

The prolonged operation has raised fears for the mental health of the miners, who are trapped in a hot shelter 700 meters deep inside the mine in northern Chile that collapsed on August 5.

For 17 days, there was no sign they were alive. Then on Sunday a small probe drill broke through and reached them, and they sent up messages saying all had survived and were overjoyed at the thought of being rescued.

The eight-centimetre-wide drill hole has been reinforced and since Monday was being used to deliver water and nutrients to the miners.

It was also serving as the sole channel of communication, with an intercom system functioning since yesterday.

“The umbilical cord is ready,” Mr Sougarret said. “Now comes the engineering design, the topography – and then begins the work of drilling.”

Engineers plan to use an industrial-sized hydraulic bore to initially make a shaft 33 centimetres in diameter.

That will then be expanded to 66 centimetres – wide enough to extract each of the men, one by one.

“It would take around half an hour to bring out each man,” said one expert, Pedro Ramirez.

Negotiating layers of rock and drilling in a way to prevent another collapse in the unstable mine were challenges that could drag out the operation over the rest of the year.

The miners said they were in good health and had water but were hungry after surviving on emergency rations of two tablespoons of tuna fish and half a cup of milk every 48 hours. Water had been trickling into the mine, too.