Water

Water fills pools in the Jungle's rooms and the Black Market's, and the lake of a flooded level. The player swims in it: it slows his falls, ends a fall without its damage, and lets him kick up toward the surface. Some monsters change in it, and some only hunt a player who's in it. Lava is, to the code, a kind of water too, but what it does is its own page's.

The clip shows a fall into a pool: the player walks off a ledge high above it, and has been falling for more than the 16 steps that would hurt and stun him on landing, when he goes into the water unhurt. He walks along the bottom, breathing out bubbles, then kicks up to the surface and climbs out ^[3].

When the middle of the player goes into water, it splashes, and from then on he's swimming ^[4]. A Cape closes in water ^[5].

While he's swimming, a fall doesn't count: the time he has been falling goes back to nothing each step, so a fall into water never ends in a long drop's stun or lost hearts, however far he came ^{[6] [7]}. Sinking, he keeps only half his falling speed each step, where in the air he keeps all of it; otherwise, with his head under, he keeps 0.9 of his speed up or down ^[8]. He walks along the bottom as on any floor. While he swims, he breathes out a bubble each time a timer of 20 steps runs out, and the step it's set again on makes that one every 21 steps; a bubble rises until it leaves the water ^{[9] [10]}.

The jump key kicks the player up whenever there's water just above his head, on the bottom or not: a jump's push, as on the ground, and 3 pixels per step more speed upward besides ^{[11] [12] [13]}. Without water above his head, the key works as on land: a jump if he's standing on the bottom, no kick if he's afloat with his head out. So a player rises by kicking from just under the surface, and the last kick carries him out onto a bank, as in the clip. Out of the water, he stops swimming ^[14].

Water stays where it is until a block around it is destroyed. Then, each step, every block of water that has empty space, nothing solid and no water, to its left, to its right or below it goes, dripping, until none does: a pool whose side or floor is blown open drains out through the hole ^{[15] [16] [17]}. Water never flows anywhere: it only goes.

Level generation makes water from the Jungle's room tiles:

The Black Market's rooms have the same tiles ^{[22] [23]}. In custom levels water is the level editor's tile w ^[24].

• Fish. Each block of water outside a block of earth gets a Piranha with a 1 in 30 chance, or on a graveyard level a Dead Fish ^[25]. Both go after a player who's swimming, and only then ^{[26] [27]}.
• Bats don't wake for, or chase, a player who's swimming ^[28].
• Stomping. The stomp check most monsters share needs a player who isn't swimming, so landing on a monster in water doesn't stomp it ^[29].
• Monsters in water splash and fall at a third of their gravity, 0.2 rather than 0.6, in the code most monsters share ^[30], and a Fire Frog that goes in turns into an ordinary Frog ^[31].
• Rubble slows in water, and a drip of water ends in it ^[32].

• A drop into water is safe from any height: a pool at the foot of a long shaft is a way down without losing hearts.
• Getting out takes a bank no higher than a jump from the surface; from the bottom, kicks bring the player up to it first.
• Swimming keeps Bats off, but draws Piranhas.

• Only water in the top 512 pixels of a level drains, except in a custom level, and none drains in Olmec's Lair ^[33]. A safety count stops the draining after 100,000 blocks have been looked at ^[34].
• Every block of water is set up with the type "Water", which the draining code reads to tell it from lava's ^{[35] [36]}.