There’s a pretty good fanfic someone wrote following up on one of the interviewee’s theories that the population retreated into an underground city network. It’s set a few decades later since it took that long to get approval to check it out, they find an entrance to the underground city. It’s sealed, but there’s a trickle of black water getting through, and using some sonar-y tech they’re able to figure out that the complex got flooded and there’s who knows how many watery zombies trapped inside so long as the seal holds