This.

Plenty of people think they're "smart" - and assume they know better than a vague warning sign: When some people see a "Danger confined space" sign they're going to think the danger is banging your head on the low-ceiling; that's not the real danger of confined spaces: a bigger threat is unbreathable gasses, like CO2, that tend to pool at the bottom of confined spaces: that's often the real danger. But yet I've never seen a sign saying "Danger: confined space: CO2 pooling risk" or similar.