Exploring Hinagdanan Cave: Biodiversity Survey and Underwater Mapping in Bohol

Beneath one of Bohol’s best-known tourist attractions lies a largely unseen aquatic ecosystem. In July 2015, a joint team from Marine Conservation Philippines (MCP) and the Silliman University Angelo King Center for Research and Environmental Management (SUAKCREM) travelled to Hinagdanan Cave to document the animals living in its underground pool and investigate the submerged passages extending beyond its main chamber.

For the MCP team, the expedition offered a rare opportunity to combine biological research with underwater exploration. Local freedivers had reported that another pool might be connected to the cave’s main chamber through a submerged tunnel. Although our primary purpose was scientific, there was considerable excitement about what might lie beyond the visible pool—and whether the cave might contain species specially adapted to life underground.

Before entering the cave, the team prepared carefully for the unusual working conditions. The volunteers’ activities were restricted to the accessible main pool, where they could work without entering submerged overhead passages. Exploration beyond the main chamber was undertaken separately by the appropriately trained and equipped technical diving team.

Our first task was to snorkel around the main pool and familiarise ourselves with its layout. Even during this initial inspection, we identified several possible submerged passages. These would later be assessed by the technical divers before any further exploration or mapping was attempted.

The fine sediment covering parts of the cave made visibility a particular concern. A single misplaced fin kick could disturb the silt and rapidly reduce visibility. Some passages were also too narrow for divers to travel side by side. These conditions meant that access had to be carefully controlled and that much of the cave system remained beyond the safe limits of the wider team.

Surveying Life in the Cave Pool

The biological survey was designed by the resident science officers from MCP and researchers from SUAKCREM. It formed part of a broader assessment of Hinagdanan Cave funded by the Japan International Cooperation Agency (JICA) under the Sustainable Environment Protection Project for Panglao.

The purpose of the survey was to document the animals inhabiting the cave’s aquatic environment and contribute data to a wider report on its biodiversity, physical conditions and appropriate management. This information would help decision-makers balance the cave’s continued use with the need to protect its distinctive ecosystem.

Our principal method for sampling small fish and aquatic invertebrates involved placing 30 homemade baited traps across the bottom of the main pool. A two-person team carefully established the sampling pattern while the rest of the group checked, prepared, labelled and baited the traps.

The traps were positioned at regular intervals across different sections of the pool. This allowed the researchers to sample areas with varying depths and levels of natural light. Some traps were placed in relatively shallow, partially illuminated water, while others were positioned in deeper areas of complete darkness.

We began seeing results almost immediately. Crabs and fish entered some of the first traps before we had finished deploying the remainder. Once all 30 traps were in position, they were left overnight to collect a representative sample of the animals moving through the pool.

Exploring and Mapping the Submerged Passages

While one part of the team conducted the biological survey, the technical divers began investigating submerged passages leading away from the main pool. Their work revealed that the visible chamber was connected to a network of underwater tunnels, air spaces and smaller chambers.

For many of the volunteers, watching the technical divers disappear into previously undocumented passages was one of the most exciting parts of the expedition. Nevertheless, the reasons for restricting access quickly became apparent. The combination of narrow passages, overhead environments and easily disturbed sediment made these sections unsuitable for divers without the necessary training and equipment.

The exploration team documented the direction and dimensions of the submerged passages so that this information could be incorporated into a map of the cave system. Several small openings could not be safely explored using backmounted scuba equipment and remained possible routes for future investigation in sidemount configuration.

Collecting and Documenting the Animals

In addition to the baited traps, divers used scoop nets to collect animals that were unlikely to enter the smaller trap openings. Captured animals were photographed and documented before being released back into the cave pool.

The following two days were spent retrieving, emptying and resetting the traps. Each animal had to be handled carefully, documented accurately and returned to the water as quickly as possible. The traps also needed to be redeployed at approximately the same time and in the same positions each day to keep the sampling process consistent.

This repetitive work provided an honest introduction to the realities of biological field research. Exploration may be the most dramatic part of an expedition, but reliable science depends on patient sampling, careful record-keeping and consistent methods. Much of our time was spent photographing crabs from every conceivable angle so that the researchers could later compare their physical characteristics with identification guides and scientific literature.

From Field Observations to Scientific Results

During the expedition, our preliminary observations suggested that the pool contained several species of crabs and fish. However, identifying animals in the field is not always straightforward. Closely related species may differ only in small physical characteristics, so the final identifications could not be confirmed until the photographs and survey records had been examined more carefully, and samples were studied under our microscopes.

The relatively small number of species did not mean that the cave was ecologically unimportant. Underground aquatic habitats are extradinary environments, with very limited light, unusual water conditions and restricted sources of food. Animals that persist in such places usually have highly specialised relationships with their surroundings.

The analysis continued long after the team left Bohol. Staff and volunteers spent many hours organising survey records, examining hundreds of photographs, confirming species identifications and contributing information to the final report.

Spending several days working in a hot, humid and frankly rather smelly cave was physically tiring, but it gave MCP’s volunteers a valuable insight into genuine field research. It also demonstrated how biological surveying and specialist diving skills can be combined to investigate environments that would otherwise remain largely inaccessible.

Most importantly, the expedition produced baseline information about Hinagdanan Cave’s aquatic life, water conditions and submerged passages. This provided a stronger scientific foundation for decisions about tourism, water quality, visitor management and the long-term protection of the cave.

We extend our sincere thanks to SUAKCREM and JICA for making the wider survey possible, and to Philippine Fundivers for providing tanks and weights for the duration of the expedition.