The Temple of Minerva Medica

In one of central Rome's less touristy sections - standing alongside the tracks leading into Termini railway station - sits what has become known as the Temple of Minerva Medica. It has been known as this since the 16th Century, but it was never a temple of any kind.
It is a 12-sided building and it used to have a concrete dome. There is evidence of underfloor heating - which led some to think it was part of a baths complex - and the walls were veneered in marble, with statues placed in niches around the interior. Unlike the Pantheon, the dome was made of thin panels of concrete attached to brick ribs. The roof survived until the 19th Century when it finally collapsed. The design was found to be too weak quite early on, as heavy buttresses were built into the structure within 20 years of its construction.

It is now widely believed to be a dining pavilion - part of a larger summer residence built by the Emperor Gallienus (253 - 268 AD). His horti (gardens) lay in this area, and he moved his entire court here each summer. Some 6th Century AD statuary was found embedded in Medieval walls built into the pavilion - suggesting the building was still in use at this time. With a busy road and tram-line on one side, and the railway tracks on the other, it is a remarkable survivor, which features in very few guidebooks.