I came across an image on Twitter which shows the Colosseum, but also the ruined fountain that used to stand next to it, the Meta Sudans. Here it is (click to enlarge):
The tweeter had found it online “somewhere”. Fortunately it is not too hard to locate: this is Prospetto dell’anfiteatro Flavio … detto volgarmente il Colosseo, 1703, by Alessandro Specchi (1668-1729). There is even a page about it here at the University of Munich, complete with an image, although, infuriatingly, a low-quality one.
I was unable to locate online any quality image, so this is about as good as it gets for the Meta Sudans (obtained by zooming my browser at this site):
It clearly shows the Meta Sudans at twice the height that we see in photographs from the 19th century. The upper section must have become very ruinous, and been demolished. No doubt some Italian source could tell us when and why this happened; but such knowledge has not reached me.
Delightful to see another picture of this vanished monument, all the same.