Minimal surfaces spanning wire frames. Mean curvature H = (κ₁+κ₂)/2 = 0 everywhere.
Catenoid: The only minimal surface of revolution. x=(a·cosh(v/a))·cos(u), y=(a·cosh(v/a))·sin(u), z=v.
Plateau (1873) verified Lagrange's 1760 conjecture that soap films are minimal surfaces by physically dipping wire frames in soap solution.