Visitors per year: 1 million
Problem: The tower had to close in 1989 because of flimsy foundations - built on waterlogged clay and sand - rather than tourist congestion.
Solution: Reopened in 1996 after being strengthened with steel cables, but visitor numbers were restricted.
Taj Mahal, India.
Visitors per year: 720,000
Problems: Tourists rub their hands on inlaid flowers on tombs in the crypt, causing the growth of a fungus that blackens the marble. Local industry and traffic create smog that stains the building yellow.
Solution: Remove heavy industry from the vicinity.
From Schoon/Popham (1995)
