The volcano may have been formed when the sea level was lower, during the last ice age[5] There is a 77 hectares (190.3 acres) soda lake in the middle of the island, occupying two intersecting craters 39 and 69 meters deep as determined by echo-sounding.

At 13 sites around the lake, large calcareous reefs extrude from rocky points; they are submerged for at least 23 m, are 1 to 2 m thick with very steep walls, and their tip emerge by about 50 cm at the end of the dry season.

Their structure alternates between layers of in vivo calcifying Pleurocapsales cyanobacteria and of red algae (Peyssonnelia sp., Lithoporella sp.

), often separated by accumulations of gastropod fecal pellets settled in cyanobacterial micrite — although the red algae are present only in the first 1 cm of the reefs.

colonizes the reefs surfaces, intertwinning with the green algae; and a dense population of Oligochaeta (worms) lives in the black sandy mud on the lakeshore.

The upper one is 22.8 m thick, it is oxygenated; compared to seawater, it is less saline but more alkaline, has a higher pH, and lower concentrations of Ca and Mg.[19] The 22.8 m deep chemocline is marked by a sharp decrease of the pH, which indicates an increase in pCO2 (partial pressure of carbon dioxide): from 340 ppmv (part per million by volume) at the surface, it rises to 240,000 ppmv at the bottom of the lake.

[19] Kempe & Kazmierczak 1990b suggest that organic matter falling into the lake is progressively respired at depth, releasing isotopically light CO2.

In surface waters, saturation index (SI) for calcite — which is a calcium carbonate polymorph, CaCo3 — is above 0.8 and that for dolomite is above 2.8; both values decrease rapidly with depth, and undersaturation is reached below the chemocline.