Built in 27 BC, the Pantheon was altered in the 2ND century BC and its appearance has changed over subsequent centuries. The colonnaded facade originally stood higher so the the dome (at more than 130 feet in diameter, the widest every vaulted with stonework) was practically invisible from the approach. Once inside the church, this added an element of surprise to the effect of the first view of the dome, stunning in the simplicity of its proportions: it is a perfect sphere, held in a cylinder of equal height and radius. In 1878 it was declared a tomb for Italy's kings and Vittorio Emanuele I and Umberto I are buried here.
Quoted from Knopf Guides.
Practically the oldest intact building in Italy, it was built as a pagan temple. Over the centuries a host of religions worshipped here and today Christian services are being held on a regular basis. It is a gorgeous building, very awe inspiring, a favorite of most visitors. This was my 3rd visit and I don't think I'll ever get tired of visiting the Pantheon.