When do you see a gelatinous cube?

Question

The gelatinous cube description reads:

Transparent (Ex): Gelatinous cubes are hard to see, even under ideal conditions, and it takes a DC 15 Spot check to notice one. Creatures who fail to notice a cube and walk into it are automatically engulfed.

Does this mean if you do not succeed on the DC 15 spot check the cube basically has greater invisibility as you cannot see it?

Or was this simply intended to be a spot check to notice it and not walk into it, and that it would be clearly visible once combat began?

Answer 1

No. It doesn't have greater invisibility (unless the DM likes unnecessary things). Transparent just means you can see through it. It is intended to be more of a trap and an ambush. It is like quicksand but terrifying. They aren't invisible, it is like looking at a tinted window or an Instagram filter. The flavor text gives some good clues for spotting one.

Inorganic material remains trapped and visible inside the cube's body. MM3.5

Players can see it by swords, armor, coins, or furniture that floats inside it. If they are unwitting metagamers, they may think it is a zero g room.

The thing is slow and huge. Combat may not be necessary. You can use one to block off a room or tease the adventurers by putting a magic sword in it. When the adventurers know what they are looking at, it won't be impossible to fight and they won't have to keep rolling spot checks.