The Sand Pyramid is inverted in the inside (it's actually an octahedron), so the more you explore the deeper you go.
Then what's the point of the upper part of the pyramid? Well, it's actually a giant vessel that is full of sand and has enough of it to fill the lower part. If someone survives to all the traps and enemies in the pyramid and manages to reach the bottom, they can claim the treasure from the sarcophagus, but as soon as they do, sand will start falling from the cracks on the walls, floors and ceilings and sandfalls will activate, quickly filling room by room. They have limited time to escape until they get trapped.
You can treat the sand as difficult terrain.
Take into account the height of each room, the adventurers may get trapped there and get suffocated or crushed on the ceiling.
After they successfully escape, the sand goes back into the top half and refill the pyramid ready for the next explorer.