One of The Strangest Places on Earth: The Chocolate Hills in The Philippines

A Landscape That Looks Almost Unreal

Early in the morning, the island of Bohol looks almost unreal. Gentle mist hangs low over a landscape of perfectly rounded green domes, rising and falling like a frozen ocean swell. At first glance, it feels peaceful—just another quiet tropical valley waking up to the day.

But hidden beneath this calm scenery is a geological secret: these hills are the exposed remnants of an ancient coral world, lifted from the sea and sculpted by millions of years of tropical rain.

Their scale is absurd. There isn’t a dozen of them, or even a few hundred—there are almost two thousand near-identical domes spread across an area larger than a major Australian city. And the strange thing is, almost nobody knows why they look this perfect. Even fewer people realise that landscapes like this hardly exist anywhere else on Earth.

What happened here matters, because the process that built these hills didn’t just shape Bohol—it created one of the most symmetrical karst landscapes on the planet, complete with caves, underground rivers, and a hydrological system that entire communities rely on. Geologists call this phenomenon tropical karstification of a Pliocene coral limestone platform, producing what are known as mogotes.

Walking Among Legends and Perfect Domes

Once you step back from the neatness of the Chocolate Hills and start walking among them, the illusion of simplicity quickly fades. The valleys between the domes are wide and perfectly flat, used for rice farming because their slopes are gentle and their soils are deep. But the domes themselves rise steeply, almost abruptly, as if the land beneath them has hardened into something different from the plains around them. For decades, this abrupt contrast fuelled legends of giants, wars, heartbreaks, and long-lost battles. The people here have stories for everything. Some say a warrior cried so long for a lost love that the earth swelled into thousands of small hills. Others say two giants threw boulders across the landscape for days until exhaustion forced them to stop, leaving the ground littered with debris. When you’re standing among these flawless dome shapes, those explanations almost feel more plausible than geology.

But the real story is far more dramatic. The Chocolate Hills began at the bottom of the sea.

The Formation of The Chocolate Hills

During the Pliocene, millions of years ago, this part of the Philippines sat beneath shallow, warm tropical waters teeming with coral reefs. Layer upon layer of coral skeletons, red algae, and shells accumulated into a thick limestone platform, a vast submarine plain of white carbonate sediment. Thin sections of the rock around Bohol still reveal tiny fossil fragments: broken pieces of Acropora coral, bands of red algae, little spirals of foraminifera that once drifted in the ancient sea. This was not a quiet environment—tectonic plates were colliding across the region, squeezing, folding, and raising seabeds toward the surface.

Over time, this coral platform was lifted out of the ocean. What had once been a thriving reef was suddenly exposed to air, sun, and rain. The uplift was uneven, causing the limestone to tilt and bend into gentle folds—broad anticlines and synclines that would later become the framework of the island’s interior. Where the rock arched upward, rainwater flowed quickly across it. Where it dipped down, water pooled and infiltrated slower. These subtle differences would eventually decide the shape of every hill and valley.

Once exposed, the limestone faced a new enemy: acidic rainwater. Tropical environments are perfect for chemical weathering. Rain absorbs carbon dioxide from the air and from the soil, turning into carbonic acid, weak but relentless. This acid seeps into every crack and pore of the limestone, dissolving it grain by grain. Bohol receives up to three metres of rainfall a year, delivered almost evenly across all seasons. This means the land is dissolving almost constantly, with no seasonal pause and no freeze-thaw cycle to reshape the surface more violently. Instead, the shaping process is slow, steady, and remarkably consistent.

At first, the limestone dissolved along weaknesses: joints, fractures, tiny pores where shells or coral fragments had once been. Underground rivers formed, carving tunnels through the subsurface. Water pooled in low-lying areas and sculpted wide valleys. Over time, the hills began to take shape—not as isolated peaks but as the leftover ridges between areas where the rock dissolved faster. The gentle domes we see today are all that remains of a once-continuous limestone surface.

Why the Hills Became Perfectly Symmetrical

But why are they all the same shape?

That question baffled early geologists. In other parts of the world, tropical karst landscapes produce jagged towers, sharp ridges, or uneven, chaotic slopes. Cuba’s mogotes often have cliffs at their bases. China’s karst forms needle-like spires. But Bohol? Bohol produced a landscape that looks like an architect’s sketch repeated a thousand times with uncanny accuracy.

The secret lies partly in the limestone itself. The rock here is poorly consolidated—almost chalky—yet filled with impurities and patches of clay. These impurities give the limestone strength in some places and weakness in others, setting up a pattern of differential dissolution. On top of that, the upper few meters of limestone are especially porous. Rain doesn’t just run over the surface; it infiltrates into the rock, dissolving it from below the soil layer. This process, known as crypto-corrosion, is subtle but incredibly effective. Instead of carving sharp cliffs or undercut notches, it rounds the hills from the inside out. Over thousands of years, slopes smooth into curves. Peaks flatten slightly. Valleys widen. Everything becomes symmetrical because the process acts everywhere at once, evenly and continuously.

Climate helps too. Bohol sits in a region where the monsoon winds shift direction throughout the year. From November to March, rain comes mainly from the northeast. From May to September, the winds reverse, bringing moisture from the southwest. This means no single side of a hill faces more rain or more erosion than the other. Both sides get soaked evenly. Both sides dissolve at roughly the same pace. The result is an entire region of hills so symmetrical that some slope profiles measured in the field are almost perfect mirror images of one another.

A Living Landscape Still Shaped by Water and Time

Human eyes are drawn to patterns, and the Chocolate Hills deliver one of the most striking natural patterns on the planet. A single hill looks interesting. But when you climb up to one of the designated viewpoints—especially near the town of Carmen—and see hundreds of domes stretching off into the distance, the impact hits hard. It feels mythic. Ancient. As if you’ve wandered into a lost landscape from a different geological era.

Yet the story doesn’t end with their formation. The Chocolate Hills are part of a living landscape, and beneath them lies one of the most complex underground systems in the archipelago. The porous limestone absorbs rainfall quickly and channels it downward into a network of caves, tunnels, and subterranean rivers. Some of these caves are enormous. The Carmolaon 2 cave drops nearly 150 metres beneath the surface and extends into passages lined with flowstone. Other caves, like those near Antequera, are filled with underground rivers where water levels can rise suddenly after heavy rain.

Springs emerge at the bases of many hills, releasing water that local farmers depend on to irrigate their rice fields. Some springs flow with such consistency that they’ve been incorporated into elaborate traditional irrigation systems, where water is guided across terraces through hand-dug channels. These systems are so old that archaeological evidence suggests people have been tapping Bohol’s karst springs for centuries.

Deforestation and Eco System Pressure

But with human habitation comes pressure. Deforestation has stripped much of the island’s original rainforest. Although the slopes of the hills once supported thick vegetation, only a tiny fraction of that forest remains. Without this natural shield, rain erodes soil faster. As the soil thins on hilltops, water infiltrates more quickly into the limestone, sometimes overwhelming the underground drainage. A disrupted balance in a karst landscape doesn’t take long to manifest as surface collapse. And Bohol has felt that impact.

In 2005, a series of collapses opened in the region of Mayana and Jagna, destroying dozens of houses and forcing more than a hundred families to flee their homes. Entire patches of land subsided, taking roads, crops, and buildings with them. The disaster wasn’t random. It was a sign of how sensitive the Chocolate Hills region is to changes in land use. When vegetation is cleared, when hills are mined for limestone, or when groundwater levels drop too rapidly, karst reacts quickly—and sometimes violently.

Despite these challenges, the people of Bohol remain deeply connected to their land. In 1998, the Philippines declared the region a National Geological Monument, acknowledging its global significance. UNESCO has also recognised the Chocolate Hills as a landscape of exceptional scientific and aesthetic value. Protection efforts have strengthened over the years, but the balance remains delicate.

When you stand among these hills, it’s impossible not to think about time. Every dome is a reminder of processes that unfolded slowly, imperceptibly, over hundreds of thousands of years. Each curve was carved by raindrops. Each valley widened by the steady seep of water disappearing underground. Each cave shaped by invisible rivers flowing in darkness. And all of it—every shape, every slope, every perfectly rounded profile—would vanish without the chemistry of limestone, the consistency of tropical rain, and the stability of a climate that favoured slow dissolution over dramatic upheaval.

The Chocolate Hills are the present-day surface of an ongoing geological story. They are the remnants of an ancient coral world, the evidence of tropical karstification operating under ideal conditions, and a demonstration of how landscapes evolve when water, rock, climate, and time interact with perfect balance. In the language of geology, they are mogotes—residual limestone hills shaped by differential dissolution and crypto-corrosion. But in the language of storytelling, they are something even better: proof that the Earth’s most improbable landscapes are shaped by forces so subtle and patient we barely notice them—until they produce something extraordinary.

Here's the video we made on this on the OzGeology YouTube Channel:

Studies Used To Construct This Article:

A MYSTERIOUS KARST: THE “CHOCOLATE HILLS” OF BOHOL (PHILIPPINES) 

Karst landscapes and karst features in the Philippines: