For that matter Rome itself was the defacto capitol of most of Europe and North Africa with a population of well over 1 million by the time of Christ. By the time of Justinian I its population was in the tens of thousands at the most and many of those were there to salvage marble, statuary, columns, etc., for building projects in other cities.
Osgiliath? 
Pripyat, near to the Chernobyl plant, had a population of 50,000.
What about Phnom Penh? Wikipedia claims it had a population of nearly 2 million at the time it was sacked by Khmer Rouge. Within days the Khmer Rouge had forcibly evacuated the city of its entire population.*
*I’m sure some remained, but the vast majority were pushed out into the countryside.
How about Babylon? At one time the largest city in the world. Now, nothing.
Valdez, Alaska was destroyed in the 1964 Good Friday earthquake, and rebuilt in a new location:
I came in to mention Angkor, but I see a couple of other posters have covered it. Angkor flourished from the 9th to the 14th centuries:
Some estimates place the population at over a million, but a conservative 750,000 is still impressive for what became a “lost city.”
Tenochtitlan was mostly knocked down–other cities throughout the world were built atop ruins. But the people didn’t go anywhere. Yes, disease took its toll–but many survived. And parts of the “design” of the Old City remain.
Ostia Antica, Rome’s seaport, was abandoned over several centuries. Of course, its marble was a “resource” for later builders.
My pick: Mohenjo-Daro supposedly had a population of only 35,000 at its peak. But the entire Indus Valley civilizition was lost until the 1920’s.
What an interesting post, I wasn’t aware of that. It answers a question I often think of while watching movies. You often see vast tracts of plainly deserted, derelict buildings and I wonder how you can have a property vacuum like that - surely someone owns it and wants an income out of it. But no people means no income.
Don’t forget Ostia, the seaport of Rome. It wasn’t destroyed in a major disaster, but silted up gradually. It’s since been excavated by archaeologists (I’ve been there). It’s not on the list of Lost Cities above.
There’s also the Roman town of Glanum in Southern France.
then there are all those towns in Egypt and Mesopotamia and Turkey – far too many to list.
Miore recently, I recall passing towns in Utah that were completely flooded out, and the magazine Weird New Jersey lists towns that have disappeared in New Jersey. The magazine iosn’t entirely trustworthy, but it’s hard to ignore pictures of abandoned buildings.
Eden, which once contained 100% of the world’s human population. 
Some of the NJ towns were abandoned because either the valley they were in is now lakes or reservoirs by plan or they were built around long dead industries like Bog Iron and Bog Cedar and some fishing villages. None of these could be considered a city. They were villages and hamlets in most cases. Most states have lost or abandoned towns for similar reasons and of course, the Rust Belt has many diminishing cities as noted.
Fascinating thread, thank you HeyHomie for starting it,
Jim {Great Magazine and two fun books now}
Ditto, Troy.
Regarding Ephesus – where is the modern Turkish city of Efes in relation to the ruins of Ephesus? I thought it was the same city continued through time, but perhaps Efes was builf on top of or near Ephesus?
Has the 3 Gorges Dam in China displaced any large cities?
Hmm…let’s see…a few that spring to mind.
Petra is the first one I think of.
Hmmm…while not actually one city, the carved underground cities of Cappadocia, Turkey seem to fit into the catagory nicely.
There is no modern Turkish city called Efes, it’s the Turkish name for Ephesus. (It’s also a Turkish beer.) There’s nothing there but ruins – the nearby town with the hotels, restaurants, etc, is called Selçuk.
Does anyone know anything about Gelonus? From what I can make of this Wikipedia page (written by someone with a shaky command of English) it seems like the ruins describe a huge city, but I can’t find any estimate of its peak population, or in fact very much on the Web at all that isn’t in Ukrainian.
Oh, and I had no idea Angkor Wat has such an enormous population. I got a little smarter today despite myself.
Yes, but don’t ask me what their names are or what the population figures are. I was watching a program about it, and got really upset when I saw that several historical landmarks were going to be lost forever because of that damned dam.
The city of Ys may or may not fit into this Thread, but the circumstances make a definitive answer problematical.