Figure 1 - Trapping vortices.

Magnetic field lines (black) can only penetrate a superconductor through a vortex, which has a non-superconducting core surrounded by supercurrent flow (red). The movement of vortices causes energy dissipation (finite resistance), but shallow pits (and mesas) on the lead surface serve as defect centres to pin the vortices in place, ensuring that current can flow without resistance.