Lying in a smooth plane
level, even, smooth, plane, spread out, extended, stretched out, prostrate, horizontal, low, low-lying, on a level, fallen, flattened, collapsed, deflated, punctured, prone, supine, recumbent, lying down, reclining, two-dimensional, flat as a billiard table*, flat as a pancake*; see also level 2, smooth 1.
Antonyms rough*, raised, uneven.