Something glides slowly and smoothly along the corridor, unseen and almost unheard. Ahead of it lie grimy and dusty tunnels; behind the passageway has been swept clean, even the mildew and mould has been devoured. Ahead the adventurers creep forward cautiously, eyes peeled and ears pricked for the first sign of hostile activity.
Phlorx!
The gelatinous cube is a quintessential part of the dungeoneering experience: as vital as the pit trap; the 10' × 10' square room guarded by an orc; the warren of narrow corridors. As far as monsters go there is absolutely nothing else like it in the book.
Physiology
The term 'gelatinous cube' is something of a misnomer: gelatinous cubes do not contain gelatine. Like the substance after which they are named, however, their bodies are colloidal. They are gels in the true sense of the word: solids dispersed in liquid.
As recently as six years ago, gelatinous cubes averaged ten feet along each edge, although lately the species' average size has increased to fifteen feet along each edge (average volume 3,375 cubic feet) and weigh 50,000 pounds. On average, the density of a gelatinous cube is 14.815 pounds per cubic foot (237.311 kilograms per square metre); just over a quarter of the density of water.
Naturally, this density is not constant throughout the gelatinous cube's body. The density varies according to the concentration of solid matter within its body. The cube is densest at its corners, then its edges, then its faces, and is usually at its least dense (and most liquid) towards its centre. This variation allows the ooze to keep its distinctive cuboid shape, and by concentrating and dispersing its mass the gelatinous cube is able to move. The gelatinous cube propels itself forward by a kind of controlled toppling: by agitating its lower surface the cube partially liquefies, allowing it to glide along. Internal convection allows the gelatinous cube to crawl forward slowly, and to extend pseudopoda when needed.
Gelatinous cubes, being lighter than water, can float when immersed, but have yet to develop any serious talent for swimming. This could be attributed to their complete lack of arms, legs, fins or any other readily identifiable appendages. There is little information on how cubes behave in water; being nearly transparent themselves, it is possible that total immersion renders a gelatinous cube almost invisible.
Gelatinous cubes are even harder to see underwater: it takes a DC 20 Spot check to notice one.
Like other oozes, gelatinous cubes are simple multicellular organisms. They have no easily discernible organs or limbs, although they are capable of manipulating their bodies to a modest degree by altering their density. extending pseudopoda to attack their food.