Boulder

Verified against the spelunky.ai port v0.1.10 on 27 October 2026.

The Boulder guards the Mines' Golden Idol. The idol sits on an altar under a giant stone tiki head; 100 steps after the player picks it up, the head opens and drops a boulder two blocks across, which rolls off toward him, smashing through blocks. It kills by crushing: one that lands on the player, or rolls into him, takes every heart.

Picking up the idol on a Mines level shakes the screen and sets the nearest giant tiki head going: 100 steps (3.33 seconds) later it opens and drops a boulder from its mouth ^{[2] [3]}. The idol only does this once ^[4].

The boulder falls and, the first time it's on the ground and still, sets off toward the side the player is on at 4.5 pixels per step ^[5]. It moves twice its speed a step in the open, 8 and 10 pixels on alternate steps at first, because both the game's pass over moving platforms and its own step move it ^[6]. On the ground it loses 1% of its speed a step, and stops once it's below 0.5 pixels a step; it turns back at the edges of the level, and edges off a block it's balanced on ^[7]. It's kept active wherever it is, rather than only near the screen as most things are ^[8].

The clip shows the player standing with the idol under the head: the boulder drops onto him and crushes him, then rolls on to the wall.

The boulder hurts nothing directly: it's solid, and kills what ends up inside it. A player whose middle is inside it, because it fell or rolled onto him, is crushed, losing every heart, and is counted in the end-of-game statistics' crushed row ^[1].

Rolling, it smashes the blocks in its way, as long as it's going at least 1 pixel a step and the block isn't the floor under it, taking any spikes on them too, and loses 0.1 pixels a step of speed for each. An unbreakable block, such as the level's border, or any block once it's slower than 1 pixel a step, turns it back at half its speed instead ^[9].

Only in the Mines, in the idol room: each side room off the path through the level, except in its bottom row of rooms, has a 1 in 10 chance of being it, once the roll for a Kali altar fails, and a level has one at most ^[10]. The room has the giant tiki head near its top and the idol on an altar below it ^{[11] [12]}.

The idol does something else in the other areas: in the Jungle the floor around it crumbles, and on a graveyard level the Ghost comes; in the Temple it drops the Ceiling Trap ^[13].

• Leave the room's floor. The boulder rolls along the ground toward where the player was when it landed. Dropping down a shaft, as in the second clip, or climbing out of its way, lets it roll past; it stops at a wall, or once it has slowed.
• Don't stand under the head. It drops straight down from the head, onto whoever is still by the altar after 100 steps.
• Bomb it. A Bomb's blast that reaches the boulder destroys it, leaving rubble and, for each of three pieces, a 1 in 3 chance of a Rock ^[14]. Nothing else breaks it: to the code that destroys blocks, it's unbreakable ^[15].

• Arrow Traps fire at a boulder rolling past, as they do at anything that moves in front of them ^[16]. In both clips one fires as it goes by.
• Standing on it. Its top is a platform: a player standing on a rolling boulder is carried along ^{[17] [18]}.

• Its step has a commented-out end that would have turned a stopped boulder into a static one, an object the game doesn't have ^[19].
• The boulder's destruction of blocks tries to remove their decoration tiles too, but the function it uses never finds a tile in GameMaker 8.0, so the decoration stays behind ^[20].