Between latitude 9 deg 21 min. and 9 deg 32 min. North, and longitude 125 deg 34 min East. The lake is situated slightly below the small peninsula running north from the eastern side of Mindanao. Clustered around it are the municipalities of Alegria, Mainit, Sison and Tubod in the province of Surigao del Norte and the towns of Jabonga, Santiago, Tubay and Kitcharao of the province of Agusan del Norte.

About 42 km. North of the lake is Surigao City and approximately 82 km. South of it is the City of Butuan. Lying beyond its eastern coastline are the Diwata mountain range including Mt. Kabatuan and Mt. Mabaho. On the western border is the southern ridge of Mount Tendido that extends up to the town of Tubay and walls the lake in the east but drops steeply to the coast.

A large and deep freshwater lake on a narrow plain between mountain ranges. It is the fourth largest lake in the Philippines and the second largest in Mindanao. It is fed by twenty small rivers and creeks but has only one outlet, the Calinawan River, which flows southward to join Aciga River and forms the Tubay River which eventually flows out into Butuan Bay. It is eutrophic with high primary production despite a low standing crop, indicating high turnover. The lake is clear with 1% of the indirect sunlight penetrating to a depth of 13 m indicating that primary production is distributed throughout a substantial depth of the water column. The lake water is greenish and odorless with an average transparency of 3.0 meters. The coastal substrate is sandy mud. The maximum depth is 223 m and the mean depth is 128 m. It is thus the deepest lake in the Philippines.

Type II. The data for the lake is characteristic of the south-eastern coastal areas of the country which show a clear trend toward heavy rainfall and low temperature from November through March and a combination of drier weather and higher mean temperature during other months particularly June through September.

The lake can be reached by taking public transport at the overland terminal in Surigao City. The travel time is about 45 minutes.

The dominant genera of phytoplankton are Anabaena, Lyngbya, Synedra, Cryptomonas, Peridinium, Gymnodinium, Melosira, Navicula, Nitzschia, and Spirogyra. Shallow areas are dominated by Hydrilla verticillata, chara sp., and Cladophora sp. Floating macrophytes include Pistia stratiotes, and Eichornia crassipes. Important terrestrial species found in the catchment are Oncosperma tigillarium and Bambusa Sp.

Bird species identified include Dendrocygna arcuata, Bubulcus, Aythya fuligula, Ixobrychus cinnamomeus, Megalurus alustris, Geopelia striata and Lanius schach.

: All of the above fish are caught, with the native gobies predominating, followed by tilapia and common carp. The most widely used fishing methods are gill-net, beach seine, scoop net, fish trap, pole and line, spear fishing and crab pot. The land around the lake is intensively cultivated and there were two timber and mining companies that operated in the lake'' catchment.

Gastropods and bivalves are collected as foods items. Sand and gravel from the lake shore are hauled and sold as housing or as road filling materials.

The Mamanwas (Kongkista) are ethnic group which have been roaming around the mountains of Surigao and Agudan but have been driven to the honterlands by Christian settlers.

About 4 kilometers north of Lake Mainit is a smaller lake called Mahukdam. Beyond the mountain ridge on the lake's western border is the Bohol sea which leis directly north of Butuan Bay.

The fishery resources are very important and about 24 economically important fishes are found in the lake. The lake is used as a feeding area by some waterbirds.

The mouth of the lake outlet is increasingly becoming shallow as a result of silt deposition in the area. The shallow mouth prevents rapid exit of the water consequently flooding the lower portions of the lakeshore including the ricefields.

Erosion from the denuded mountainside where timber and mining companies operate threatens the watershed area as well as the lake.