It has many islets around a central lagoon . This is about 25.1 kilometres ( 15.6 mi ) ( Nâ `` S ) by 18.4 kilometres ( 11.4 mi ) ( W-E ) , centred on 179 Â 7 â E and 8 Â 30 â S. A It comprises numerous islets around a central lagoon that is approximately 25.1 kilometres ( 15.6 mi ) ( Nâ `` S ) by 18.4 kilometres ( 11.4 mi ) ( W-E ) , centred on 179 Â 7 â E and 8 Â 30 â S. An annular reef rim surrounds the lagoon , with several natural reef channels .