U.S., Aug. 16 -- Visiting a concentration camp or radiation zone may not seem like everyone's idea of a holiday but dark tourism sites are drawing millions of people around the world every year.

Here is the ultimate visitor guide to 8 dark destinations you won't want to miss.

National 9/11 Memorial & Museum

Where: New York, USA

History: The National September 11 Memorial is a tribute of remembrance and honor to the 2,977 people killed in the terror attacks of September 11, 2001 at the World Trade Center site and at the Pentagon, as well as the six people killed in the World Trade Center bombing in February 1993.

Visitor info: The 9/11 Memorial is free and open to the public daily from 7:30 am to 9 pm. Museum tickets can be purchased ...