The topography, composition, and water characteristics of the shallow platform surrounding Johnston Island were studied by aerial observation, diving, sounding, and sampling. The platform consists of highly permeable calcareous sediments and rocks similar to those of atolls. The peripheral reef, however, extends along only the leeward margin of the platform. Incompleteness of the reef may be due to partial erosion of an atoll during a glacial epoch of colder water and lowered sea level or to tilting of the atoll so that an original windward reef is now submerged, leaving only the present leeward reef above sea level. Refraction of waves around the platform and against the leeward reef produces a wave-driven current that moves easterly against both trade-wind and general oceanic current.