The world’s longest building is a never-used Nazi-era resort called Prora which would stretch the entire width of Manhattan and across the Hudson River to New Jersey. Built by Hitler on the Baltic Sea to provide recreation for the masses, the buildings were completed and are still standing, but the resort never opened. Just under 3 miles long, it has capacity for 20,000 guests and is nearly impossible to photograph in its entirety because it shrinks to a hairline in any comprehensive view.
The buildings lay vacant from 1939 to 2011 apart from sporadic, partial use by the former-East German military and now have historic landmark designation. Finally in 2011 a youth hostel opened, occupying one-eighth of the structure, and in the 2010s about half the remainder was sold to private investors for renovation. In 2015 the first few renovated apartments were sold; half the complex remains unused government property.
Although it clearly is one of the foremost examples of grandiose Nazi architecture, Prora is not particularly well known and I’m not sure why. Perhaps because it’s not in a city, but it’s not extraordinary remote either. Also, if you want to split hairs, it’s not technically the world’s longest building, because its eight blocks are separated although closely spaced.