Phantom Glass

A special glass that turns non-solid — and walkable through — while you're looking at it.

Being "observed"

Looking directly at a Phantom Glass block (line of sight, no carved pumpkin on your head) makes it non-solid. Any other Phantom Glass directly connected to it (orthogonally adjacent) also turns non-solid, so a whole connected wall or floor of Phantom Glass opens up together, not just the one block you're looking at.

Solidifying again

Once nothing is watching a connected group anymore, it solidifies again after a brief delay (with a bit of a particle/sound cue), so don't count on stepping away from view and having it stay open.