Jupiter tempel in Baalbek
Remains of the Jupiter temple in Baalbek, Lebanon, June 2005.
This was the largest place of worship in the Roman Empire and the ruins are surely impressive. It was completed AD 150.
Only six columns are still standing. There used to be 54 (a square with 10x19) of these huge columns that are 23 m high and 2.2 m in diameter. They are among the biggest in the world. It is hard to get a grip on how enormous this building must once have been.
In the same area is also the Temple of Bacchus that is almost as impressive.
The temple area is on Unesco list of World Heritage and you can read about it here
(Requires Google Earth)