What is a Japanese tatami mat made of?

Traditional tatami mats have a core of compressed rice straw, chosen for its humidity control, thermal insulation, and durability, topped with a woven rush grass (igusa) surface. This build is what gives tatami its firm yet slightly springy feel. The bottom of the tatami mat is lined with linen fabric or the same rush grass as the top layer, never blue plastic or polyurethane.