Given the following reasoning and answer, what was the question? The relevant information is: Vespasian's decision to build the Colosseum on the site of Nero's lake can be seen as a populist gesture of returning to the people an area of the city which Nero had appropriated for his own use.
The answer: the site of Nero 's lake
The question Passage: Although the Colossus was preserved, much of the Domus Aurea was torn down. The lake was filled in and the land reused as the location for the new Flavian Amphitheatre. Gladiatorial schools and other support buildings were constructed nearby within the former grounds of the Domus Aurea. Vespasian's decision to build the Colosseum on the site of Nero's lake can be seen as a populist gesture of returning to the people an area of the city which Nero had appropriated for his own use. In contrast to many other amphitheatres, which were located on the outskirts of a city, the Colosseum was constructed in the city centre; in effect, placing it both symbolically and precisely at the heart of Rome.

Question: Based on this passage, what was the colosseum built on top of?