Curiosity finally drove me to Edinburgh to visit Mary King’s Close. We found the site, located opposite St Giles Cathedral on the High Street, paid our money and with another eighteen people, followed a guide down the steep steps into darkness. Mary King's Close turns out to be four narrow streets with tenement houses that in the 1600s stood seven storeys high. The inhabitants lived dirty, nasty lives and died by their hundreds in the plague years, and our guide described it all in gory detail. It is not a tour for the faint hearted or the light of stomach!
In 1753 houses at the top of the closes were knocked down and part of the lower sections were kept and used as the foundations for the Royal Exchange (now the City Chambers). Since the streets ran from the High Street down the slope to the Nor Loch, the buildings that remain at the lower end of the slope are still several storeys high under the floor of the Chambers.
The lighting is so dim as to be non-existant, so I cannot say I really "saw" the closes. The lighting level in the photo from the website is what you might expect, and you can find out more from the website: http://www.realmarykingsclose.com/home.htm
But the experience of groping around in the dark, up and down stairs and through cellars and byres is unique, and I'm glad I went and saw for myself.