The Turtle Islands, in the Sulu Archipelago, is located in 6.07683°N, 118.31386°E. It is bounded by the Sulu and Mindanao Seas on the west and north, and by the Celebes Sea on the east and south. It is an archipelago with a rocky islet Sibaung and six (6) small islands – Taganak, Baguan, Langaan, Boan, Lihiman, and Great Bakkungan.

The Turtle Islands has a total aggregate area of 242,967 ha (242.97 sq km), including its surrounding waters. Land area within the six islands totals 318 ha (3.18 sq km). Topography ranges from flat, e.g., low hills and plateaus, to high features, e.g., as high as150 masl. Each island has unique topographic characteristics.

The Turtle Islands is located at the southwestern edge of the Sulu Sea, near the northeast edge of the continental shelf of Borneo. It is situated in the southwestern edge of what is called the Celebes-Sulu Block. This tectonic block encompasses eastern Palawan, the Sulu Sea, northeastern Borneo (Sabah) and part of Sulawesi. This block is wedged between the Eurasian Plate and the Philippine Sea Plate. But the origin of this block including the adjacent Banda Block is not well understood. These blocks are interpreted either as an exotic block (crustal block that rifted and drifted into its present location) or fragments of the adjacent major plates (Rangin and Silver, 1990). These blocks are estimated to have been incorporated with the Philippine arc during the Miocene time (5 to 22.5 million years before present).

Interpretation of geologic evidences shows that this region has been stable (no major deformation) since Middle Miocene (15 million years before present). This allowed the deposition of thick layers of geologically young sediment. However, volcanism persisted until the Plio-Pleistocene epoch as indicated by the presence of Plio-Pleistocene volcanics in the Turtle Islands, Cagayan de Tawi-tawi and the submerged Cagayan Ridge.

The Cagayan Ridge is a submerged ridge elongated along the northeast-southwest direction parallel to Palawan and the Sulu Archipelago. This ridge divides the Sulu Sea into two basins. Result of drilling done at the southern part of the ridge yielded volcanic rock samples of 5 to 1.5 million years old (Plio-Pleistocene age; Rangin and Silver, 1991). This ridge is marked by a number of platform reefs, namely, Tubbataha, Basterra, Bancoran and San Miguel. Drilling in Tubbataha revealed a 400-meter reefal deposit.

Analysis of the Quaternary geology of the region showed a fluctuation of sea level during the last 15,000 years before present. The result of the study indicated an oscillation of sea level from 100 meters below the present sea level to a height of 5 to 50 meters above the present sea level (Tijia, et al., 1986). The interpreted fluctuation of sea level in the Southeast Asian region generally agrees with the general global trend.

With the recession of sea level during the glacial period, the Turtle Islands was landlocked and was likely part of the Pleistocene land bridge from Borneo to the Philippines. Shoreline during this period must be 25 km away.

With the rising of the sea level at the end of the glacial period, the Turtles Islands must have been drowned when the sea level rose 50 meters above the present sea level. This probably explained the mature topography of the Turtle Islands as well as the almost uniform elevation of the hills that make up the various islands.

Volcanism in the Turtle Islands was initially thought to be active due to the occurrence of mud volcanoes. Eruption of the mud volcano in Lihiman is periodic, occurring almost monthly, effusing mud that lasts for about five days. Upwelling is accompanied by explosion sound and earthquakes with intensities ranging between scale 1 to 3 in the Rossi-Forel Scale (PHILVOCS).According to PHILVOCS, the mud effusion may not necessarily be that of a volcanic eruption, because of the absence of several characteristic phenomena.

With this geologic history, the evolution of the island as a turtle rookery must have started only about 5,000 years before present.

Although no detailed pedological studies have been conducted, observations in Taganak and Baguan Islands show that island soil is dull red due to the presence of ferric oxide. The humid layers are noticeably thin particularly in Taganak Island (IPAS Technical Survey, 1992). In most part of the municipality particularly along the coastal areas, the soil is sandy. See Geophysical characteristics per island for a detailed discussion on the substrate of each island is in.