Nemo's Voyage Slot by Light & Wonder

The clever bit sits to the right of the reels. A glowing blue Depth gauge runs down beside the grid with the Nautilus submarine riding it, and every spin nudges the craft lower while the pressure needle creeps right. As the sub crosses into each deeper section, a stronger wild feature switches on and stays on for that level. The first lock turns all wilds into x2 or x3 multipliers, and when two of them land on the same line their multipliers add together rather than multiply, so x2 plus x3 makes x5. Drop another section and you get Pressurised Wilds, where a wild that forms a win floods every position beneath it on its reel with more wilds. Deeper still are Clumped Wilds, which drop a whole block of adjacent positions in as wild.

The bottom section is the one worth chasing. Down there every spin turns one to three whole reels fully wild, and that level carries the best returns in the game. An Increase Pressure symbol on reel 5 shoves the needle several ticks at once, so you can plunge through the tiers faster than the one-tick-per-spin drift. Max the pressure out and the Nautilus ascends instead, switching each feature back off as it rises past the locks, then the descent starts over. Wilds only appear on reels 2 through 5, never reel 1, so a combination always has to begin on that leftmost reel without help.

The free spins ignore the depth meter entirely. Three Bonus scatters, which land only on reels 1, 3 and 5, hand over 8 spins with no retrigger. A pink Giant Squid wraps its tentacles around the grid and rips off positions, turning them into Persisting Wilds that stay locked for the rest of the round on reels 2 through 5. The depth gauge simply freezes when the bonus opens and picks up from the same spot afterwards.

It is dressed as a Jules Verne brass-riveted submarine, the 5x4 array bolted into an ornate copper porthole against deep teal water, with Captain Nemo's portrait, a hammerhead shark, a blue whale, an angler fish trailing its lure, a harpoon and a treasure map filling the reels. A WMS machine now flying under Light & Wonder, and the depth-meter progression is what keeps it from feeling like a standard 40-line lines slot.