More than 1,500 people are expected to visit the Clearwell Caves near Coleford in the Forest of Dean for the launch of its Christmas display on Saturday.
Staff at the caves have spent the past three months decorating the caves with a series of models and scenery, to depict a winter story about a polar bear.
The story was written by Heather Wright, the wife of Jonathan Wright who is managing director of the tourist attraction.
It tells of small boy who finds a hungry polar bear and decides to keep it as a pet. The story will run this year and also next Christmas.
Running through a series of caves and alleys, the display also features a Santa's grotto – each child who visits the caves will receive a free present – a Christmas market and a cafe area.
And the Forest of Dean Brass Band will serenade visitors to the attraction this weekend from 2 to 4pm on Saturday and Sunday.
Mr Wright, aged 52, has worked at Clearwell Caves for since it became a tourist attraction under the management of his father Ray in 1968.
He said: "This year's display is the best I've seen and we really hope people are going to like it.
"We first did a Christmas event 25 years ago. This year's is really good. All the display has been supplied locally and set up by us so we're really proud of it."
The Clearwell Caves attract some 50,000 visitors each year and also host a museum about the mining of iron ore which used to take place at the caves.
Mr Wright said: "My father had been a founder member of the Royal Forest of Dean Caving Club and he knew that there was a whole world under the forest that people were unaware of.
"He wanted to open it up so that people could know what the cavers were seeing and so that's what started the attraction that we have here today.
The Christmas Fantasy runs from 1st to 24th December, 10am to 5 pm daily. For more information go to www.clearwellcaves.com.