The phenomenon where a material becomes perfectly diamagnetic below a critical temperature is the Meissner effect.
When a material enters its superconducting state, it expels magnetic flux from its interior rather than merely conducting electricity without resistance. Persistent screening currents flow near the surface and oppose the applied magnetic field, producing near-zero magnetic field deep inside the superconductor.
Walther Meissner and Robert Ochsenfeld discovered the effect in 1933 while studying superconducting tin and lead. Their observation showed that superconductors are more than perfect conductors: magnetic-field expulsion is a defining property of superconductivity. This is why a magnet can be repelled or levitated above a cooled superconductor.
A common mix-up is treating the Meissner effect as ordinary diamagnetism or simple electromagnetic induction. Superconductors exhibit perfect diamagnetism in the Meissner state, but the effect arises from superconducting screening currents. The magnetic field does penetrate a thin surface layer called the London penetration depth, so “perfect” describes the bulk idealization rather than every point in the material.