The story of the transformation of Welbeck enters upon a new stage with the succession, in 1854, of the Marquis of Titchfield (William John Cavendish-Scott-Bentinck) as fifth Duke, born in 1800.  He it was who designed and had constructed the mysterious underground apartments and tunnels for which the Abbey and its environs are famous.  There were miles of weird passages beneath the surface of the earth, one tunnel alone being nearly a mile and a half in length, stretching towards Worksop, while others ran in various directions.

Welbeck is nearly 4 miles from Worksop, and a stranger on approaching the Abbey is likely to receive a mean impression of its vast extent.  The architecture is a mixture of the Italian and classical styles, and its having been built at different periods, with so many of its adjuncts underground, makes it wanting in imposing features.

In various parts of the estate about 50 lodges were erected for the occupancy of gardeners and keepers.  They were of Steetley stone, all similarly planned and pleasing to the eye, what there was of them above ground; but the Duke had subterranean kitchens made at the side and lighted them with bulls’-eyes at the top.

He spent about 100,000l. a year in the development of his plans, and employed as many as 1,500 workpeople in helping him to gratify his hobby.  When it is remembered that his reign as Duke lasted a quarter of a century, from 1854 to 1879, it will be seen that artisans of all descriptions found Welbeck a veritable gold-mine.  Even so late as November, 1878, a Nottingham newspaper correspondent, on visiting Welbeck, was impressed with its appearance as that of the premises of “some great contractor who had an order for the building of a big village.”  There was the buzz of machinery, large areas were covered with bricklayers’, masons’ and joiners’ sheds, wherein any new mechanical contrivance was put to the test.  For more than eighteen years the vicinity of the house resembled a builder’s yard, in the centre of which the Duke lived and moved and had his being, enjoying, in his way, the piles of bricks and mortar surrounding him.  After he had decided upon the erection of a new building he had a model of it made for his inspection, and if approved of, it was proceeded with.

Any tramp or wayfarer who applied for work at Welbeck was put on the staff, and the market value of his labour paid.  The Duke seemed to find grim pleasure in the society of the casuals who made their way to his stone-yards.

The wing built by the Countess of Oxford in a former generation had a new storey put to it, with a magnificent suite of 14 new rooms furnished in Louis XIV. style, richly gilded, and with mantelpieces of white marble.

An underground passage was made leading to the old riding school, built by the Duke of Newcastle in 1623, but since converted to other uses, such as a library and church, after the erection of the new riding school.  Beneath it are great wine cellars with subterranean communications.