Pantheon
The Pantheon, one of the most impressive buildings
in Rome, was rebuilt in A.D. 117-125 by Emperor Hadrian to replace an
earlier temple designed by Agrippa in 27 B.C. The original
rectangular temple, consisting of sixteen single granite columns,
became the porch for the newly devised dome structure. Once the
Romans learned to use concrete, they were able to mold on the ground
their rounded tops for their buildings and hoist them into place on
top of a rotunda. The Pantheon is just such a structure with 
an opening at the center top of the dome.
Because this opening is not covered, the floor is slightly concave
with a drain at the center so as to catch the rain when
necessary.
The Pantheon was originally dedicated as a temple to all gods, hence the name.
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