The huge room measuring 106ft (49m) by 63ft (19m) was built by the 5th Duke of Portland, who also had a series of tunnels constructed.
It’s understood that it was originally intended as a chapel but was used as a picture gallery and ballroom.
There is also a suite of five rooms next to it, constructed to house the Duke’s library.
Although described as underground, these rooms are not covered by earth or lawn and are more accurately described as below ground.
Their flat roofs and skylights are visible in aerial photographs, although at ground level they are concealed from most directions by shrubbery.
The Duke also made alterations to the house, including new bathrooms and plumbing, and new lodges were built at entrances to the park.
This provided work for thousands of men and the Duke, who got on well with his employees, earned the nickname the ‘workman’s friend’.
One of the underground tunnels going to Worksop was a mile and half long and was big enough to take a horse and carriage.
Another tunnel led from the house to the riding school, and another was used just by workmen.
Some of the tunnels’ skylights can be seen from the Robin Hood way footpath.