Along one of Halcyon's coastal plains, a few minute's dive from the surface, a massive bubble serves as a cap to a natural crater. The barrier itself is quite permeable to visitors: Generators are hidden in the walls, keeping the interior seperate from the outer. A slight push is all it takes for the bubble to yield, letting any guests into the interior.
The crater's interior has gone through quite the cozy makeover. The walls and floor are done in reflective obsidian, polished to a mirror shine. Aquamarine and amethyst run in swirling, chromatic veins through the periphery. The club is layered into two 'rings'- a upper floor, about seven feet from the 'roof', and the central ring, reaching all the way down to the bottom of the crater.
Benches and tables line both tiers, arranged in a booth-shape. Globes sit at each, radiating various colors, changing every few moments to another hue. Along the upper ring, a bar juts out of the wall, offering various drinks- A little bit of packaging is necessary to imbibe underwater, but it's all taken in stride. The atmosphere seems to be a little relaxed, though the center is designated as a dance floor. In truth, it'd be described better as a dance cube, or cylinder...Regardless, opposite the bar, there's a pair of turntables and other equipment, and some trance blares through speakers hidden in the walls- mostly Neptunian local, and as such, the locals would always say that for a 'real' appreciation of the music, one needed a bit of psionic resonance. If you ask the bartender nice enough, you could probably get a drink to facilitate such capabilities- Neptunians love to share their culture!
Finally, and probably, most importantly, is the fact that- despite the bubble-barrier- the club is entirely submerged. Why the barrier is there, then, is unknown: Possibly to prevent noise pollution, or mayhaps to keep a transformative agent in the water inside? For those who are not able to breathe underwater, there is a rack with artificial gills, in choker shape, provided- free of charge.