The Pantheon was built in 27 BCE by Agrippa. It´s located in the heart of the city, located between Piazza Navona and Piazza Venezia, and considered to be the best preserved antique monument in the city and the oldest standing domed structure in Rome
The Pantheon was originally built as a temple to "all the gods" of Ancient Rome.

Sources, related links:
Wikipedia - Pantheon, Rome