The Yap (pronounced Wop or Uop) group (9° 25′ and 9° 46′ north latitude, 138° 03′ and 138° 14′ east longitude) consists of four principal islands, Yap or Rull, Gagil or Gagil-Tomil, Map, and Rumung, and about ten small islands. These are located on one triangular submarine platform and surrounded by a fringing reef. The reef varies in width from one-half to 2 miles. The islands enclosed by the reef make a compact group 16 miles long and 8 miles wide. The total land area has been estimated to be about 38½ square miles, whereas the reefs enclose about 10 square miles of lagoon.
These islands represent the top of a great underwater ridge, similar in formation to the island arcs of Palau, the Ryukyu, and Japan. Like them they rise from the continental shelf and west of the sial line, and the eastern side of Yap is marked by a deep trench in the ocean floor. The core of the island group is of metamorphic rock, but the eastern portion is composed of weathered lava. The greatest elevation, and consequently the greatest erosion of this ridge, was on the eastern side. Subsequent subsidence has resulted in the ocean invading the old erosional valleys. Coral growth has been unable to keep pace with the subsidence of the valleys, and as a result these estuaries form natural channels throughout the reef today. Tomil, the commercial harbor, is on the southeastern side of the group. Although it is the best harbor of the group, it has a narrow, dangerous entrance. The coral-free channel of 6 fathoms depth is only 100 yards wide. Yap-Colonia, which appears on some maps as Yaptown, is the chief administrative center of the Yap Islands and is located on the western side of Tomil Harbor.
The largest of the individual islands, Yap, is about 12 miles long and 3 miles wide. The highest elevation of the whole group, 585 feet above sea level, occurs near the northern end of Yap. These northern hills lose elevation toward the south, and most of the southern portion of the island is low, with occasional mangrove swamps.
Yap and Gagil-Tomil were almost separated, except for a narrow neck of land, by the subsidence that formed Tomil Harbor. This separation was completed when the Germans dug the Tageren Canal.