This is the Shaykh Luqman Mausoleum in Sarakhs, on the Turkmenistan-Iran border. It was built in 1356, and is now being restored. Getting access to themuqarnas (stalactite vaulting) right up in the top of the dome involved squeezing up between the double shelled walls of the lower part of the building; then an exposed and hair-raising shuffle around the outside of the base of the dome; before re-entering the dome itself to join the craftsmen working there. [C.M.2]
