International expedition “Mulu 2019”

An international expedition of speleologists and scientists conducted research into some of the largest caves in the world and their underground ecosystems. The expedition, organized by members of the NEPIAST Speleo Club – Burgas and the Bulgarian Federation of Speleology, took place from 1 to 26 July 2019 in Mulu National Park, Sarawak Province, Malaysia. The purpose of the expedition was to study caves in the karst area of Mulu National Park in Malaysia.

Mulu National Park has been declared a World Heritage Site in order to protect unique nature and geological phenomena. The presence of limestone and sandstone combined with the huge amount of rain have formed unique karst forms, including some of the largest caves in the world. In the park can be observed both surface karst forms – wells, canyons, nets, etc., and underground: caves, springs and underground rivers with impressive size.

Limestone rock formations in the park

More than half of the vast biodiversity of the island of Borneo can be found in the park – the seventeen different plant zones support 2142 non-flowering plants and at least 2000 species of flowering plants, including 182 orchids and 15 species of carnivorous plants. The animals currently known from the park include 116 species of mammals, from which 54 are bat species, 305 bird species, 100 species of reptiles, 97 amphibians and 48 fish species. The abundance of invertebrates is estimated at about 20,000 species, including 360 species of spiders, 147 species of dragonflies, 281 butterflies, approximately 3,000 species of moths, 458 ants and more than 4,000 species of beetles. Some unique plant species are endemic to the inaccessible rock formations of the park.

Cave formations

The purpose of our expedition was to explore a new potential environmental service – the contribution of cave bat colonies to nutrient input into terrestrial and aquatic ecosystems – and use the results to promote cave conservation and mitigate human-nature conflict. Bats guano contains essential nutrients such as nitrogen and phosphorus and is a major source of food for a wide variety of cave organisms. When underground rivers pass through caves, they transport guano to adjacent surface habitats. With such large aggregations of bats in caves, the contribution of nutrients can be significant, especially for tropical soil poor nutrients; however, this link has not yet been thoroughly investigated. Caves and bats face a number of threats due to increased tourism, extraction of materials, quarries, direct killing and loss of habitats. Understanding the importance of bats colonies for nutrient cycles in tropical forests will help to determine the consequences of their reduction or elimination for these systems. To extrapolate our findings globally, we work in caves in two tropical forest sites located in Costa Rica and Malaysia.

An endemic cave crab from the caves in Mulu

We chose to work in Mulu national park because of the huge underground caves that shelter millions of bats – great conditions for our research. The area has been surveyed for several decades by British cavers, which has made our task easier and we have been able to navigate and select the most appropriate caves from which to sample. We were impressed by Mulu Park’s attitude towards the caves, which are being which are considered great assets – something that we hope to see soon in Bulgaria. Nature lovers from around the world gather in the park to visit some of the caves and watch one of the most spectacular phenomena in the animal kingdom – the exodus of over 2 million bats from Deer cave every evening.

Our guide in the Sarawak chamber – the dimensions of the hall cannot be captured in one photograph

During the expedition we explored 5 caves from 3 large cave systems: Clearwater system with Clearwater and Wind cave, Deer cave system with Deer cave and Deer water cave, and Good luck cave with the largest cave hall in the world – “Sarawak chamber ”. We took samples from the underground rivers and sampled the vegetation around the cave entrances. We also visited several caves that turned out to be unsuitable for exploration.

The entrance of the Deer cave

We did the main work in Deer cave, because the conditions there were optimal – there is an enormous colony of free-tailed bats (Chaerephon plicatus) – 2 milion bats, and there are two rivers that flow through the cave. Inside the cave is situated one of the largest underground halls in the world, there are two entrances and two rivers that flow through it – one river that enters the cave from the upper entrance and is lost in a siphon, and a second river that flows from a rock cleft and flows to the lower entrance. We spent over a week sampling vegetation, bat guano and groundwater. Taking into account the weather forecast, we took water samples several times – in dry weather and after rain. We made several trips to the “upper” entrance of the cave, as well as to the place where the first underground river appears on the surface.

Cristian Castillo at the entrance of the Deer cave, waiting for the bat exodus

The Clearwater system is one of the longest cave systems in the world – 227 km mapped length. A large river flows through this cave, and comes to the surface in the form of a huge karst spring, which flows into the navigable river Melinau, the main mode of transport for the local population. We visited the cave twice to take samples of vegetation. In order to take samples of the cleanest possible water, we entered a few kilometres along the underground river. The main inhabitants of the cave, in addition to bats, are a species of swift that specializes in inhabiting caves, go deep underground and orient themselves by emitting clicking sounds. Their nests are a highly valued delicacy and can reach a fabulous price. Poachers go around the side paths of the park in search of nests of speedsters from the caves, so free movement outside the tourist routes is not allowed.

A bat from the Hipposideridae family

The Wind cave is part of the Clearwater system, and the connection between the two caves is made by the water of the underground river. The entrance to the cave can be reached only by boat. We visited this cave several times to collect samples. The water in the cave in some places is very deep and there is a danger of a flood when raining, so we were extremely careful at each visit. Again, in the cave, in addition to bats, we observed a large number of swiftlets.

Petyo Savov and Angel Ivanov measure the flow of the underground river in Wind cave

The cave Good luck shelters the largest cave hall in the world – the Sarawak chamber. The cave could be reached after several hours of hiking through the rainforest. We visited this cave only once because of the difficult access and the danger of rain. Sarawak chamber is reached after climbing upstream an underground river, the level of which rises by several meters after a rain. In some places around the cave we saw rope parapets, which during our visit were a few meters above us – a sign of how dynamic the water flow is. To reach Sarawak Chamber we climbed a very long scree, which gave the feeling that we were climbing a mountain peak. The Sarawak hall is so huge that the lights of our headlights and torches did not reach the walls of it and it was difficult for us to feel its real size. The main inhabitants of the cave are swifts, katydids, spiders, and few bats.

The trekking to Good luck cave includes the crossing of over 10 rivers
Water sampling in Clearwater cave

Mulu National Park is strictly guarded – the permit we had did not allow us to move without a guide, nor to carry out research or exploration other than our project. However, we were happy to be able to complete the tasks we had set for ourselves, because bureaucratic difficulties are hampering much of the research and development in the area, and our project was considered more than successful.

Dr. Gloriana Chavery and Dr. Maria Sagot observe bacterial colonies in the Wind cave

During our stay in the park there was a tragic accident with tourists – during a flash flood at the upper entrance of the Deer cave the water dragged a group of 8 tourists and killed a Dutch tourist and a local guide. The search for the bodies of the two victims lasted more than a week. One body was trapped deep into the Deer cave siphon and had to be removed by scuba divers. This event was very upsetting for our team and park staff and hindered our work. We stopped working for a week and tried to help the management of the park as much as we could.

This is the surfacing point of the Deer water cave river

We are currently processing the samples and analysing the results of the study. Our project is extremely important for the conservation of bats, because the main arguments for their protection are always focused on the ecosystem services they provide. Our findings could potentially add another to the list. We hope to have the opportunity to work again in this unique area and to contribute to the protection of caves around the world.

Participants in the Expedition


Stanimira Deleva (NGO NEPIAST – Burgas) – expedition leader
Angel Ivanov (NGO NEPIAST – Burgas)
Nia Toshkova (ПКСУ „София“)
Violeta Zhelyazkova (ПКСУ „София“)
Petyo Savov (NGO NEPIAST – Burgas)
Boris Dimitrov (NGO NEPIAST – Burgas)
Dr. Ognyan Todorov (Regional museum of natural history – Plovdiv)
Svetlozara Kazandzhieva (Regional museum of natural history – Plovdiv)

International participants

Ferdinand Salazar (Grupo Espeleologico Anthros – Costa Rica)
Cristian Castillo Salazar (Grupo Espeleologico Anthros – Costa Rica)
Prof. Gloriana Chaverri (Universidad de Costa Rica)
Dr. Maria Sagot (OSWEGO state University of New York)
Amanda Savage (Smithsonian Tropical Research Institute – Panama)

The group in front of the base camp

Expedition “Mulu 2019” is organized by the Bulgarian Federation of Speleology, on the initiative of Speleo Club NGO “NEPIAST”, and is part of the international calendar plan of BFSU for 2019. The event is implemented with the partial financial support of the Ministry of Youth and Sports (IMC) under the “Program for the Development of Sports for All for 2019”. The expedition is co-financed by National Geographic, Bat Conservation International, The explorers club.

The entrance to the Wind cave can only be reached by boat