A cistern collapsed on the market square. You can still see the water supply very well.
The cisterns were closed off at the top with a stable round vault. The paving stones of the agora lay over it.
Intact cisterns, but also cisterns that have collapsed from time to time (e.g. Lybe-Seleukia) carry water and are very deep.
When entering an agora you always have to be prepared for collapsed or semi-collapsed cisterns.