I know for a certainty this picture was taken of the inside of the Grand Shaft whilst going down as I had no energy left for photos when I was coming back up!
The Grand Shaft is part of the Western Heights fortifications built on the outskirts of Dover in case of invasion by Napoleon. If you are interested in its history, there is a little bit about it in the Miscellany album as it's also included on today's date there as a half and half theme.