Green’s Dictionary of Slang

even further ‘down’ than the supposed situation of hell.

1895		G.A. Sala in Daily Tel. 18 July in Ware (1909) 180/2: Throughout my life I have always had a burning desire and a dogged determination to get below the surface of things, and Eugene Sue’s masterpiece took you, as the saying is, ‘down below Nathaniel’, as regards the basements and the subterranean of society.
1909		J. Ware Passing Eng. of the Victorian Era.