According to the font of all knowledge, the catacombs under Odessa total some 4,000 km stretching under the city and surrounding area of Odessa. The area has never been fully mapped (really it's barely been mapped at all). Apparently the first tunnels were built as a result of mining the soft limestone, then as tunnels were expanded they hooked into each other in a vast network that has housed everything from smugglers to Nazis (and apparently even housed a wine cellar at one point).
The tunnels are starting to become flooded, which adds an extra element of danger for your dungeoneering party. Not only do they have to navigate 4,000 km of territory (with whatever might lurk beneath), they now have to avoid flooded areas and sudden washouts.
Some pictures for inspiration:
Some early rewards for the party:
Some interior glamour shots:
I wonder where the train goes . . .
And some bits of what's been mapped.
In a place as old as this there's no telling what you're going to find.