Unfinished Cave - Ajanta Caves (India)
For me, this unfinished temple was the most amazing sight at the "Ajanta Caves" site. It really gave an idea of the enormous work that was needed to carve each temple using very basic tools, and of the process involved (i.e. in what order the digging and carving was performed).
Also impressive to see such a huge project stopped in the middle and unfinished, probably due to some calamity (war, drought, or epidemic) and never finished.