On a high volcanic rock plateau in Ethiopia stands a unique Christian holy site, Lalibela. In the 12th century, a crossed-shaped church called the House of St. George was built by cutting away the surrounding rock to leave a 12 meter by 12 meter cross. It is formed out of a single rock without a single seam. The interior was also hollowed out from rock. Lalibela has eleven stone churches, which are connected by tunnels. They were built by a ruler named King Lalibela in the 12th century.
from World Heritage Site UNESCO
Video explaining the history http://whc.unesco.org/en/list/18/video
Awesome aerial pictures of Africa (which lead to this discovery)
Travel Video on Youtube showing various angles and countryside.