There was once a claim that, with the naked eye, astronauts could see the Great Wall of China, but to my knowledge this has been retracted leaving no other contender that I know about off hand. At night, however, you can see cities and roadways as lines and splotches of light.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
for the
NASA IMAGE/POETRY
Education and Public Outreach program.