Arch of Constantine, Rome, northern side, Meta Sudans area before.

The Arch of Constantine (Italian: Arco di Costantino) is a triumphal arch in Rome, situated between the Colosseum and the Palatine Hill. It was erected by the Roman Senate to commemorate Constantine I's victory over Maxentius at the Battle of Milvian Bridge in 312. Dedicated in 315, and the latest existing triumphal arches in Rome, it was also the only one to make extensively reuse major sculpture from earlier monuments. The arch spans the Via triumphalis, the way taken by the emperors when they entered the city in triumph.

The Meta Sudans (Latin: "sweating turning post") was a large monumental conical fountain in ancient Rome.
The Meta Sudans was built some time between 89 and 96 under the Flavian emperors, a few years after the completion of the nearby Colosseum. It was built between the Colosseum and the Temple of Venus and Roma, close to the later Arch of Constantine, at the juncture of four regions of ancient Rome: regions I, III, IV, X (and perhaps II).
A meta was a tall conical object in a Roman circus that stood at either end of the central spina, around which racing chariots would turn. The Meta Sudans had the same shape, and also functioned as a similar kind of turning point, in that it marked the spot where a Roman triumphal procession would turn left from the via Triumphalis along the east side of the Palatine onto the via Sacra and into the Forum Romanum itself.
The Meta Sudans was built of a brick and concrete core, faced with marble. It seems to have "sweated" the water (sudans means "sweating"), rather than jetting it out the top. This may mean that it oozed out the top, or perhaps that water came from holes in its side. The monument is estimated to have stood up to 17m high; until the 20th century, its concrete core was still over 9m high. It had a base pool 16m wide and 1.4m deep.
The fountain was obviously damaged in the Middle Ages because it already appears as a ruin in early views of the Colosseum. Photos from the end of the 19th century show a conical heap of bricks next to the Arch of Constantine. The ruins of Meta Sudans survived until the 20th century. In 1936 Benito Mussolini had its remains demolished and paved over to make room for the new traffic circle around the Colosseum. A commemorative plaque was set in the road. Although the above-ground structure is gone, its foundations were later re-excavated, revealing the extensive substructure. After another excavation in 1997-98 the traffic circle was closed and the area became a pedestrian district.

more on https://en.wikipedia.org/wiki/Arch_of_Constantine , https://en.wikipedia.org/wiki/Meta_Sudans , http://www.roma-antiqua.de/antikes_rom/kolosseum/meta_sudans