Bubble Up!

Pufferfish act as the game's Money Symbols, carrying individual prize values that can be awarded when they're caught inside a Bubble Frame. Money symbols can also contribute to winning combinations themselves.

The clever part isn't really the Money symbols themselves — we've seen plenty of those before — but the uncertainty created by the frames. A screen containing several attractive values means very little if the bubble decides to go sightseeing somewhere else.

The Bubble Frames are Bubble Up's defining feature. Frames ranging from 2x2 all the way up to 6x6 can appear over random areas of the grid, collecting the Money symbols contained within them.

It's a good example of a feature that's extremely easy to understand without being completely predictable. Bigger frames naturally give you more room to catch Money symbols, while placement is just as important as size.

A full 6x6 frame is the one you really want to see because it covers the entire playing area — no fish escapes that particular net.

Two Bubble Frames can appear together, and any Money symbol sitting where those frames overlap can receive a multiplier of up to 10x before its value is awarded.

This is where the otherwise straightforward frame mechanic becomes considerably more interesting. You're no longer only looking at whether a Money symbol has been caught; you're looking at exactly where it sits and whether the two bubbles have crossed over it.

It's a relatively simple multiplier mechanic, but one that fits naturally with the main feature rather than feeling added simply to make the feature list longer.

Landing three to six Pearl Scatter symbols triggers Free Spins, beginning with six spins. The number of Scatters used to trigger the feature determines how many modifiers you receive, ranging from one to four.

A Bubble Frame appears on every Free Spin, immediately making the feature more active than the base game.

There's progression too. Every three additional Scatters collected during the bonus award another two Free Spins and increase the Bubble Frame by one level, eventually reaching a maximum 6x6 frame covering the entire grid.

That gives the bonus a proper sense of development. You're not merely watching six free spins disappear one by one; landing additional Scatters can make the remaining spins increasingly interesting.

Before Free Spins begin, players reveal Feature Modifiers which alter the bonus round. Depending on the number of Scatters that triggered Free Spins, between one and four modifiers are awarded.

These can increase the number of Money symbols or Scatter symbols appearing, start proceedings with a larger Bubble Frame, or guarantee two frames on every spin.

I like this addition because two bonus rounds don't necessarily begin under identical conditions. Getting multiple modifiers — particularly ones that complement each other — can make the feature look considerably more promising before the first Free Spin has even started.

Bubble Up! also attaches named Fixed Prizes to some Money symbols rather than using a progressive jackpot. The tiers include Mini, Minor, Major, Grand and the top Maxi prize.

They work well within the existing mechanic because they're not detached jackpot meters sitting above the game. A valuable prize can be staring back at you from the grid — but you still need the Bubble Frame to catch it.