The Meissner effect is a phenomenon in quantum physics in which a superconductor negates all magnetic fields inside of the superconducting material. It does this by creating small currents along the surface of the superconductor, which has the effect of cancelling out all magnetic fields that would come in contact with the material. One of the most intriguing aspects of the Meissner effect is that it allows for a process that has come to be called quantum levitation.
The Meissner effect was discovered in 1933 by German physicists Walther Meissner and Robert Ochsenfeld. They were measuring the magnetic field intensity surrounding certain materials and found that, when the materials were cooled to the point that they became superconducting, the magnetic field intensity dropped to nearly zero.
The reason for this is that in a superconductor, electrons are able to flow with virtually no resistance. This makes it very easy for small currents to form on the surface of the material. When the magnetic field comes near to the surface, it causes the electrons to begin flowing. Small currents are then created on the surface of the material, and these currents have the effect of cancelling out the magnetic field.