An open room lay before them, occupied only by a few cobwebs and dust. Upon entering, a phrase is seen on one wall. One of them utters the phrase out of wonder for its meaning, unknowingly activating the room. An eye opens on the wall in front of the poor souls and with a quick flash of light, the last thing heard from the room is heart retching screams... then silence.
Real World: some Indians in the Amazon treat their eyes with a traditional potion applied with palm leaves. Brutally painful, the drug alters vision, giving the jungle's dense green walls greater texture and dimension. You could adapt this to desert or swamps, or other hard to navigate regions.