The greatest artwork of the 20th century was 100 miles long and nearly 12ft high. A multitude of hands took 13 years to make it. They had to work fast to evade the ­border police, for they were working inside the perimeter of East Berlin. Until recently, no concerted attempt was made to document the wall. Now, German documentary film-maker Hartmut Jahn has traced fragments of the wall as far as a men's room in a Las Vegas casino and a floating restaurant in Maine...