Magnetic field lines form closed loops.
Magnetic field lines emerge from the north pole of a magnet and enter at the south pole of the magnet. This is inferred by taking a compass, and marking the direction where the compass points at different places around the magnet. When we join the points, we get a pattern of lines which emerge from the north pole and enter at the south pole. Inside the magnet, the magnetic field lies go from south pole to the north pole. Hence, they form closed loops.