Rapture used to be a nice place to live. That is, if you didn't mind being at the bottom of the Atlantic Ocean. And putting up with the occasional rants of its megalomaniacal founder, Andrew Ryan.

That was before the mutants took over, though, and by the time we learned about Rapture — in the 2007 video game "BioShock" — the city was a wreck, its art deco architectural wonders largely reduced to rubble.

In "BioShock 2" (2K Games, for the Xbox 360, PlayStation 3), which takes place 10 years later, Rapture has degenerated further into chaos. The sea has reclaimed much of the property, the mutant "splicers" have gotten more powerful and Ryan is long gone.

So, too, is "BioShock" lead designer Ken Levine, who walked away from his creation before publisher 2K Games started talking sequel. While Part Two was inevitable, given the commercial and critical success of the original, many fans have been dreading the Levine-free follow-up.

Happily, however, Rapture appears to be in good hands. New creative director Jordan Thomas, who developed one of the most memorable sequences in "BioShock," maintains the nerve-racking claustrophobia of the underwater metropolis — and expands on its political conundrums.

Ryan's Ayn Rand-inspired free-market philosophy has been undermined by Dr. Sofia Lamb, who urges Rapture's underclass to join in collective effort. (Ryan himself saw Lamb as a "Bolshevik.") There are stirrings of a religious cult, symbolized by images of butterflies. And there is a mystery surrounding the abduction of girls from the surface to serve as "Little Sisters."

The Little Sisters had an essential role in the first "BioShock," mining a vital mutagen called ADAM from corpses, and they were protected by metal-encased behemoths called Big Daddies. In "BioShock 2," you are a Big Daddy — but your Little Sister has been taken away, and you need to find out why.

The new game opens up some previously unseen neighborhoods of Rapture, including its slum, Pauper's Drop, and its red-light district, Siren Alley. Most intriguing is Ryan Amusements, a theme park in which a spooky animatronic Ryan guides children through his hellish vision of the Earth's surface.

The dark satire of that sequence echoes the tone established in the original game, and it demonstrates an admirable fidelity to Levine's original vision. "BioShock 2" doesn't feel as jaw-droppingly fresh — but it quickly develops its own momentum, building on Rapture's mythology rather than just rehashing it.

There are some technological improvements as well. The primary difference is that your character can now simultaneously wield mutant powers, like electricity or hypnosis, with more familiar weapons like shotguns and machine guns. When a gang of splicers is bearing down on your Little Sister, it helps that you can fill them with lead and set them on fire at the same time.

"BioShock 2" doesn't radically revise the formula of its award-winning predecessor. Rather, it's a beautifully executed, smartly paced sequel that left me looking forward to future visits to Rapture.