The staffer, dressed in black with a neon halo of a glow-stick necklace, doesn't look much like a lifeguard. Yet, there he is: standing next to a swimming-pool ladder, blowing a whistle and shouting to me, "Swim through! Swim through!"

It's one of many hallucinogenic moments I'll encounter at Concourse Dance Bar, the month-old reimagining of a sprawling, subterranean Center City offtrack betting parlor as a nostalgic playground — complete with an adults-only ball pit — for those recently of legal drinking age. (I brought a 25-year-old friend who told me, "What's weird is, even I feel old here.")

It can take several minutes of wandering the concrete jungle near 16th and Market to find the velvet ropes and bouncer marking the otherwise inconspicuous entrance. Within, the dingy stairwell really does recall the grim descent into a subway station, except for a few pool floats scattered along a stalled escalator. (Avram Hornik, whose other establishments include Morgan's Pier and the Dolphin, initially envisioned a two-story slide into the club. That was scotched for safety reasons; instead, clubgoers can duck into a much smaller slide from a lower landing onto the dance floor just below.)

It's not yet 10 p.m. when I arrive. The combination of '90s hits pumping through the sound system and an empty dance floor makes me feel like I'm early for a high school dance.

So I stop by the bar and ask about the cocktails. "They're all really sweet," the bartender warns. I try the jungle-juice-inspired Green Line Trolley ($9), which transports me back to freshman-year frat parties, minus the bongs, tapestries, and Che posters, and sample the Pop Rocket ($10), made with champagne, Razzmatazz liqueur, and a dusting of Pop Rocks that tend to pelt the incautious drinker squarely in the face. A Yards Philly Pale Ale seems like a better choice.

"I feel like their market research was asking a bunch of 22-year-olds what they miss about being 10: 'Ball pits. My mom's Oriental rugs,' " my friend suggests, taking in the decor, which also includes faux fur coozies on the concrete pillars.

So we channel our inner 10-year-olds and inquire about the ball pit.

"There's no cover," a staffer assures me, though that notion had not crossed my mind. But there are "pool rules": shoes off, check your drink and valuables at the desk, and wait your turn to trust-fall in. Within, it's an Instagram-filtered Hieronymous Bosch painting set in an Ikea play area. Amid the nonstop selfies, I observe a woman determinedly mounting an orca-shaped pool float, a few couples making out, and several playful/aggressive splash fights. After five or 10 minutes, a staffer blows a whistle and orders everyone out.

"Lost-and-found inquiries are high," marketing manager Rose Brownell says — so high that Concourse has created an online request form. Each Tuesday, when the pool is drained so the balls can be disinfected, they recover 10 or more cellphones.

The dance floor is comparatively calm, though within minutes a man knocks a beer all over me. He does not apologize. There is, evidently, a pickup scene. ("You look like a troublemaker," a man tells my friend, attempting to navigate that fine line between flirtation and defamation.) But mostly it's women dancing with their friends in attire ranging from slinky cocktail dresses to sweatpants — plus a scattering of guys in man buns and backward baseball hats, and one man in sunglasses and a fedora  I can only assume is an off-duty bar mitzvah DJ.

To Brownell, that's the point. "A lot of nightclubs you have to dress up,  act a certain way, there's bottle service. That's not what we're doing here. This is a place to be silly and let loose. There's no dress code. There's no pretense."

Concourse Dance Bar

1635 Market St.; 267-534-4128;

When to go: To avoid a line, arrive before 10:30 p.m., or go on Sunday, when there's no cover. It's open 5 p.m.-2 a.m. Friday (with happy hour 5-7 p.m.); and 9 p.m.-2 a.m. Saturday-Sunday. There's a $5 cover starting at 9 p.m. Friday and Saturday.

Order: Local beers on tap include Evil Genius New Phone, Who Dis? ($7), Yards Philly Pale Ale ($6), and Victory Helles Lager ($6).

Bring: Your bachelorette party. Your five-year high school reunion. Your friends who are regressing.

Bathroom situation: There can be a slight stench of vomit and, on my visit, a line long enough to provide a bonding experience between two young women who slurringly discovered they had a lot in common: They were, it turned out, both 22! Inside, a chant of "E-A-G-L-E-S Eagles!" began in the men's room, with much pounding on the walls, and quickly spilled over into the ladies'.

Sounds like: A soundtrack that felt ripped from my 1999 high school prom — think old-school hip-hop and '90s novelty dance hits — at a booming 107-decibel volume that left my ears ringing even after I'd surfaced.