Underwater Reef · Grid

Reef Diver

Twenty-five tiles on the sea floor, a handful of urchins hidden underneath. Turn tiles over one at a time to raise your multiplier, and surface with the coins whenever you have had enough.

A grid of gold-edged tiles on sunlit sand, two lifted to show coral and a sea star beneath, with bubbles rising.

Multiplier1.00×

Safe tiles0

Next tile pays—

Choose how many urchins you want, set a stake, and dive.

Urchins hidden in the grid

3 urchins 5 urchins 10 urchins

Stake in coins

Your coins 10,000

Coins are free play tokens with no monetary value. They cannot be bought, won as prizes, exchanged, redeemed or withdrawn.

How to play

Pick 3, 5 or 10 urchins and set your stake, then press Dive in.
Turn over any tile. Coral means safe and your multiplier goes up; an urchin ends the round.
Press Surface with coins to bank stake × multiplier, rounded down.
Clear every safe tile and the round cashes out on its own at the top multiplier.

Arrow keys move between tiles and Enter turns one over, so the whole grid works without a mouse.

How the multiplier is worked out

After k safe tiles with u urchins in 25, the chance of having got that far is the product of (25 − u − i) ÷ (25 − i) for each tile you turned. The multiplier is 0.99 divided by that chance — the fair value of the risk you took, less one per cent.

Multiplier after a given number of safe tiles.
Safe tiles	3 urchins	10 urchins
1	1.12×	1.65×
3	1.47×	5.00×
5	1.99×	17.51×
10	5.00×	1,077.61×

Ten urchins pays far better per tile because two tiles in five hide one. The multiplier never rewards you for luck you did not need.

The urchins are placed first

All urchin positions are shuffled into the grid before your first pick, using crypto.getRandomValues, and they stay put. No tile changes what it hides based on where you click or how well the round is going.

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