They are in... a forest.
Well, it's like a forest. It has forest-like attributes. It's more like a forest than it's like any other normal kind of place. It contains trees, and grass, and moss, and... that appears to be it, apart from the group and what they brought with them. There's no sound except the sound they make, no visible motion except their own and the occasional sway of a branch in a gentle breeze.
The ground is flat. Unnaturally flat.
The trees are unnaturally uniform in size and style, and vary only within the strict bounds of their format, like they were created by a sculptor who was beautifully talented at detail work but had only a brief verbal description of how tree trunks work and a few good samples of bark. They stand in columns as straight as fenceposts, precisely two feet wide along their whole length, and their leaves and branches look more or less like normal leaves and branches but are placed with an odd carefully-varied regularity. And, speaking of carefully-varied regularity, they're laid out in a grid. Placed at random within that grid, but if you take a moment to see the pattern, there is no tree anywhere in sight that's so much as an inch out of alignment; each trunk precisely fills its two-foot square of ground.
There's a hollow in the otherwise flat ground, a short distance away. It is aligned with the grid. The ground dips by exactly two feet, into a four-foot-by-six-foot indentation, and the borders of that indentation occupy all and only the grid squares bordering the lower part. Within those borders, the slope is irregular enough to fool a superficial glance, but like the trees, the areas of sloping ground don't stray an inch outside their invisible boundaries.