CEBU, a province sitting atop vast layers of soluble rock, has long faced a persistent geological hazard: sinkholes.
The risk became acutely apparent following a magnitude 6.9 offshore earthquake on Sept. 30, 2025, which struck northern Cebu and triggered ground collapses across the province.
This seismic event did not create the danger but significantly accelerated a natural process, forcing authorities to quickly update safety maps and relocate residents.
The problem is systemic, rooted in the island’s geology, and is exacerbated by both major seismic activity and everyday weather patterns.
What happened?
Following the earthquake that displaced thousands of families, several sinkholes emerged in parts of Cebu Province, particulalry in San Remigio, Medellin and Borbon towns.
Dr. Karlo Queaño, chief geologist from the Department of Environment and Natural Resources-Mines and Geosciences Bureau (DENR-MGB), told SunStar Cebu on Monday, Oct. 6, that many areas in Cebu are underlain by limestone, which increases the potential for sinkhole formation.
Queaño reported that the western, eastern, northern and southern parts of Cebu are predominantly composed of limestone.
Limestone is a sedimentary rock mainly made up of calcium carbonate, often formed from the remains of marine organisms like coral and shells. Over time, these materials accumulate, get compacted and harden into solid rock.
The Provincial Disaster Risk Reduction and Management Office reported 13 sinkholes in the town of San Remigio, eight in Medellin and one in Borbon. These locations are all situated in areas where the subsurface is predominantly composed of limestone.
Why it matters
The emergence of these sinkholes is a direct threat to public safety and infrastructure, transforming large sections of the island into high-risk areas for development and habitation.
The event highlights the danger of building structures in regions underlain by vulnerable rock formations.
A key concern is the longevity of the threat. As Queaño, noted, the formation of these ground collapses can continue for an extended period, potentially ranging from months to thousands of years, depending on the thickness and durability of the underlying limestone.
For residents, the primary dangers include sudden ground collapse and the threat of groundwater contamination if structures are built over these weak spots.
Measures
The MGB is updating geohazard maps to prioritize areas for tents, evacuation and relocation sites to ensure residents’ safety from hazards such as sinkholes, flooding and landslides.
The MGB has identified thousands of potential sinkholes through satellite interpretation, all of which require urgent field validation.
Queaño said thousands of potential sinkholes identified through satellite interpretation still need field validation, stressing that without on-site checks, these remain “possible sinkholes.”
The karst subsidence maps for Cebu, including a detailed field inventory, are yet to be completed.
Authorities advise residents to monitor for ground fractures and persistent puddles in low-lying areas during rainfall, as these may signal underground cavities and limestone formations vulnerable to soil collapse.
Queaño said building structures directly on top of sinkholes is discouraged, citing risks of groundwater contamination and further ground subsidence. / DPC