The Hardy Boys have been called "a cultural touchstone all over the world." Their adventures have been continuously in print since 1927. The series was an instant success: by mid-1929 over 115,000 books had been sold, and as of 2008 the books were selling over a million copies a year (the first "Hardy Boys" book, "The Tower Treasure", alone sells over 100,000 copies a year). Worldwide, over 70 million copies of Hardy Books have been sold. A number of critics have tried to explain the reasons for the characters' longevity.