It had been just shy of 15 years since the monster had come to New York City.
One day it had just come from the sea. Of course there were conspiracies, that it was a US government conspiracy, that it was a Russian one, or a Japanese one, or who ever was a convenient target to point a finger at then.
But after the city was mostly flattened, the whole world took a hit, a big one. If any government had made the beast, they must be regretting it as well. The economy tanked world wide, and even now, it's still just starting to recover.
The beast couldn't be beat. It absorbed nuclear attacks, bombs made no dents in it, and no toxin seemed to work on it. And it didn't seem to worried about leaving the city, though no one new entirely why. Getting visuals, among all the burning destruction became hard, and whatever allowed it to absorb nuclear attacks sent all our equipment haywire.
With no idea how to handle the situation, the world governments decided to do the only options b they could find. They quarantined it. A massive wall was built all around the city, on the far shores and into the sea. There were great doubts that the wall would hold the beast back, but it was presented as a band-aid, a way to give them time.
Except the monster never tried to leave. And the research didn't show anything. And the guards around the wall became more laxed. And the world moved on.
A black market struck up, people who risk the tunnels and the bridges to hunt for treasures in the city. People left, and people left fast. Some are paid to search for specific objects (or at least to claim they do), some just go looking for whatever they can fine. Urban legends sprawl far and wide, that the beast settled to breed, that the town was over run with smaller monsters, that if you made a sounds or did a certain action it would find you no matter what, that if you held still it couldn't see you...no one really knows anything. The research that has been done gets passed around again and again, but it tells little.
But there's plenty that take the risk, anyway.