they're made from flimsy fiberboard, but if you actually crack one open you'll see that all the joints are reinforced with wooden blocking, and the flat surface

they're made from flimsy fiberboard, but if you actually crack one open you'll see that all the joints are reinforced with wooden blocking, and the flat surfaces are filled with corrugated material. they're light, and if you get any water past the exterior finishing they disintigrate. but they're deceptively strong as long as all the joints are screwed together tightly.