Afghanistan's potential wealth has long been studied with interest by its ruling powers. The first scientific exploration of the land came with British invasions in 1839 and 1878, and the first systematic surveying efforts began in the mid-20th century, when French, German, Italian and Soviet geologists, at the invitation of King Zahir Shah, traveled the nation on foot and donkey-back, taking rock samples by hand. It was the Soviet invaders, though, who conducted what remain the most extensive ground surveys: They used drilling, trenching, and field samples to evaluate 20 sites in detail, paying special attention to the large Aynak copper deposit south of Kabul and the even larger Hajigak iron deposit in the Hindu Kush. After the Soviet withdrawal in 1989, all geological work came to a halt. In 1995, as the Taliban massed on the outskirts of Kabul, the staff of the AGS did manage to compile most of the previous research, and when the Taliban took Kabul a year later, the staff hid the documentation in their homes, where it remained until the current occupation.