Level sets which are one and two sheeted hyperboloids

For two-variable functions, the level  sets {(x,y): c = f(x,y)} are curves in the plane. For three-variable functions, the level sets {(x,y,z): c = f(x,y,z)} are surfaces in three-space.  This surface show the level surfaces of 

for c = -1, 1. The level sets are hyperboloids of one and two sheets.  It is designed and printed by Chloe Ham, GMU Math Makerlab.