The Irasiquero River leaves the largest freshwater lake of Timor-Leste – Irasiquero Lake – and flows towards the nearby sea. This would be geographic triviality if… the river would not disappear underground in the impressive Mainina Ponor after making half of the distance to the sea.

Karst processes

Iralalaro Lake resembles a 6.5 km long scar in the mountainous forestland in the eastern tip of Timor-Leste. The area of the lake is changing according to season and weather conditions – from 10 to 55 km2. In this area, bedrock consists of carbonates (e.g. limestone) and there are ongoing intense karst processes. Lake itself is an enormous karst depression – polje – and there are known numerous caves, sinkholes, ponors and other karst phenomena around it.

Irasiquero River and sacred ponor

The only surface outlet from the lake is Irasiquero River. It is true that this river flows exactly towards the 9 – 10 km distant sea – but there is an impressive mountain range – Paitchau Mountains – in between. After flowing for some 4.5 – 5 km the river reaches a ponor at the foot of mountains – and the stream with a roar disappears in a swallet which is less than a meter in diameter. Every second here flow 5 – 30 m3 of water.

The sea still is some 4.5 km far away. There have been done experiments with a dye tracing in Mainina ponor – the dyed water appeared in several springs on the other side of Paitchau Mountains (up to 975 m) – but also in faraway springs closer to the northern coast of Timor-Leste. This looks like a hint at the existence of an enormous underground cave system.

Diving into swallet is very dangerous – the stream is very fast and powerful even in the dry season. The bed of the ponor does not have any forest – at wet season here forms a lake which might be up to 20 m deep.

Local people consider this to be a sacred place.

Dry tropical forest

Mainina ponor is located in a unique, primeval forest – the largest remaining dry tropical forest of Timor-Leste. In this unique forest have been found new species of animals, such as turtle Chelodina mccordi timorlestensis (described in 2007 and living in the wetlands around the lake) and hardyhead fish Craterocephalus laisapi, living only in Irasiquero River.

Plans of hydroelectric power plant

Timor-Leste has a shortfall of electrical energy production facilities. There has been proposed to divert part of the water from Iralalaro Lake to the underground power plant through a 4 558 m long underground pipeline.

Closer investigation shows that the realization of this idea is very complex. Construction of tunnels through carbonate rocks nearly always brings unpleasant surprises – enormous voids, underground streams, and flash floods. Construction works might bring also changes to hydrological regime of this region, construction of new roads will dissect the primeval forest.


  1. White, Susan; White, Nicholas; Middleton, George; Report of Findings on the Proposed Iralalaro Hydro-Electric Power Scheme, Timor-Leste, January 2006. Accessed in the 22nd May 2011
Mainina Ponor on the map
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Location, GPS coordinates: 8.4737 S 127.1807 E
Categories: Sinkholes
Values: Geology, Visual
Rating: 2 out of 10 stars
Where is located? Asia, Timor-Leste, Lautém district, 4.4 km E from Muapitine village, south-east from Iralalaro Lake

Landmarks of Timor-Leste

The island country of Timor-Lest has magnificent scenery, interesting geological attractions, rich biodiversity and promising archaeological finds. This country is rather little known and in the future there are expected more discoveries.


Great Blue Hole, Belize
Great Blue Hole / Eric Pheterson, Flickr / CC BY 2.0
Category includes outstanding sinkholes – large natural depressions or holes, which for most part represent collapsed caves.

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