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In the valley of the Colosseum there is a layer of clay under many metres of
sand and silt. In prehistoric times the ground level was about 10-15 metres lower than it
is today. Rain and water from streams gathered in the valley, and, depending on the
season, formed a marsh or a little lake in the depression. From
there the waters reached the valley of the Circus Maximus and then the Tiber river.
Some drillings recently carried out have demonstrated that the site had been reclaimed since archaic times, with a drain that collected the waters as far as to the Circus Maximus Valley. The site had been certainly inhabited, at least since republican times, because remains of many buildings of that epoch have been discovered all around the valley. We know that those buildings were expropriated and demolished after the fire of 64 AD, when Nero decided to make his first residence there (the Domus Transitoria) In the place of the Colosseum there was a lake, surrounded by buildings and gardens, which we know about from ancient authors.
The images show the sites of the survey and the results obtained. The first map shows the points of the survey (the T numbers) and also the route of the underground line B, which, as one can see, winds between the Colosseum and the Arch of Constantine (T5). When the excavation for the underground was made, many ancient drains – and probably also the passage between the amphitheatre and the Temple of Venus - were lost.
The site in itself had advantages, but also drawbacks. One of the main problems was the drainage of the site, but at the bottom of Nero’s lake the engineers managed to reach the clay bed that would have supported the foundations. Drainage was guaranteed: it has been calculated that, even though it reaches such a low level, the bottom of the arena remains 10 metres above that of the drain.
In the firm clay bed an elliptical ring was excavated, 31 metres wide, 6 metres deep. Then the foundation was raised for 6 more metres, so that the thickness of this enormous doughnut is about 13 metres. By raising the foundations from the bottom of the lake, the excavation work was reduced, and was substituted with the easier job of filling up the gap with earth and waste materials of the surrounding buildings, which were demolished. Click here for a page on the foundations.
