From the book description of photographer Michael Wesely’s book Open Shutter on Amazon.com:
Since the early 1990s, German photographer Michael Wesely has been inventing and refining techniques for using extremely long camera exposures to take uniquely compelling photographs. Through the use of filters and a very small aperture, yet one that is standard in a professional camera lens, he is able to diminish the amount of light hitting the negative to the point where he can extend the exposure many thousands of times longer than we would ordinarily expect. Some of Wesely’s pictures of the rebuilding of Berlin’s Potsdamer Platz, for example, in a series completed in 1999, were continuously exposed over a period of 26 months.
The photo accompanying this post, courtesy of PetaPixel, is one of his Berlin rebuilding photos, specifically of Leipziger Platz. PetaPixel has several more photo samples, and you should absolutely check out every one.