Santa Maria dell"Arco Oscuro is a small 18th century devotional chapel in a short road tunnel by the Via delle Belle Arti, just north of the Villa Giulia and on the other side of the road. Pictures of the chapel on Wikimedia Commons are here.
The remote origins of this chapel lie in an overpass built over the road as part of the 16th century layout of the Villa Giulia. This formed a short tunnel on the road, the arco oscuro because it was long enough to be dark in the middle. The road here used to be called the Via dell'Arco Oscuro.
Here a copy of the icon of Our Lady of Divine Providence, to be found at the church of San Carlo ai Catinari, was placed in 1579, and this itself came to be regarded as having miraculous powers. As a result a small chapel was excavated from the hillside to the north of the archway, connecting with it by a doorway, and the icon enshrined here. Also a cell for a hermit was dug out on the hillside above, and the little complex was consecrated in 1797.
Unfortunately for the hermit, in the latter half of the 19th century it appears that local olive-growers were having trouble with oil that they were storing in nearby caves. It was turning rancid, and the hermit's evil eye was blamed. The ill-feeling seems to have been so bad that the hermitage was closed down.
In the early 20th century a tramway was built along the road, which is still running (it is number 19). The tunnel was bypassed, and turned into an extension of the chapel. The eastern portal was walled off, and the western one gated. It is the latter that you can see from the road.
The archway occupies a change in the course of the present retaining wall, is semi-circular and is formed of large rusticated blocks. The keystone displays the coat-of-arms of Pope Innocent XI. The archway has been left open, and is protected by a modern railing gate with a metal grille above. Within, you can see a modern mosaic of the Annunciation on the far wall.
The old chapel is through a doorway on the right, and contains many ex-votos around the original icon.