The ring current in the pi-clouds of aromatic systems enhances the external magnetic field outside of the ring and diminishes it inside the ring. Both of these effects are manifested in [18]-annulene. The outside hydrogens have a chemical shift of 9.25 ppm and the inside hydrogens show up at -2.88 ppm (yes, that is minus 2.88 ppm). Note that the molecule is completely planar!