Being scenery makes the object not only static but also not described by the game unless actually examined by the player. We can't actually get into the Square Chamber yet, though. Just because there is a map connection up from here to the Forest, it doesn't follow that there's a corresponding connection down. Really, you need a good few artifacts to take back to civilization before you can justify giving up the expedition. As is often the case with properties, instead of giving an actual message you can instead give a routine to print one out, to vary what's printed with the circumstances.
Rooms also have rules of their own.
It bubbles obscenely, distends and then bursts into a hundred tiny insects which run for the darkness in every direction. Rooms have before and after routines just as objects do, and they apply to anything which happens in the given room. This particular rule could easily enough have been part of the definition of the mushroom or the sunlight, and in general a room's rules are best used only for geographical fixtures. Sometimes the room may be a different one after the action has taken place. The Go action, for instance, is offered to the before routine of the room which is being left, and the after routine of the room being arrived in.