Arenal Volcano National Park

Listen first. Beneath the hum of insects, there is a low, steady call rolling through the trees. That is a howler monkey, and it can carry a long way through this forest. Under your shoes, the path may feel gritty and dark. That grit is old volcanic rock, broken down over decades. And above you, if the clouds allow it, a near-perfect cone rises from the green. This is Arenal Volcano National Park, in the northern lowlands of Costa Rica's Alajuela Province.

Let's start with the shape. Arenal is a stratovolcano. That means it was built in layers, one eruption on top of another, with lava, ash and rock piling up over thousands of years. The result is the steep, symmetrical cone that children draw when they imagine a volcano. It stands more than 1,600 metres above sea level. Compared with many volcanoes in the world, it is young. Geologists consider it one of the younger volcanoes in Costa Rica.

Here is the first surprise. For a long time, many people living here did not think of Arenal as a volcano at all. It had been quiet for centuries. Its slopes were covered in forest, and locals often called it a mountain. Farms and small villages spread across the land at its feet. Cattle grazed in the fields. Nobody expected what came next.

On July 29, 1968, Arenal woke up. The eruption began suddenly and violently. Explosions threw rocks and hot ash across the western slopes. Superheated clouds of gas and debris swept downhill. Two small communities, Tabacón and Pueblo Nuevo, were destroyed. Dozens of people lost their lives — the toll is usually given as around eighty-seven. Crops, livestock and roads were buried. For the families of this region, it was a disaster that changed everything.

And then the volcano did not go back to sleep. For more than four decades, Arenal stayed active. Lava moved slowly down its flanks. At night, people could sometimes see glowing rock tumbling from the summit. Rumbles and small explosions became part of daily life in the area. Visitors began to arrive just to watch. A quiet farming region slowly became one of the best-known travel destinations in Central America.

Around 2010, the activity faded. The volcano entered what scientists describe as a resting phase. You will not see red lava today. But Arenal is still considered an active volcano, and it is still monitored. That is one reason the summit is closed to hikers. The trails inside the park stay on the lower slopes, where the ground is safer and the views are often better anyway.

Those lower trails cross something unusual — old lava flows that are slowly turning back into forest. Some paths lead over flows from the 1990s. Look closely at the ground. You will see sharp, dark, porous rock, full of small holes where gas once escaped. Ferns, mosses and young shrubs push up through the cracks. Here, you are watching a forest rebuild itself almost from nothing. It is a slow process, but it is happening right in front of you.

Now compare that with the older forest nearby. Where the lava never reached, the trees are tall and heavy with life. Vines hang from the branches. Bromeliads, plants related to the pineapple, sit high on the trunks and collect rainwater. Strangler figs wrap their roots around other trees. Keep your eyes moving. Coatis, which look like a raccoon with a long snout and a striped tail, often trot along the paths. Toucans and parrots move through the canopy above.

The second surprise is about the weather. Many visitors come here and never see the summit at all. Arenal sits where moist air from the Caribbean side meets the mountains. The air rises, cools, and turns into cloud. On some days, the cone disappears completely. Then, without warning, the clouds tear open for a few minutes, and the whole peak appears. Locals often say the clearest views come early in the morning. Patience here is usually rewarded.

Turn toward the west and you may catch the shine of water. That is Lake Arenal, the largest lake in Costa Rica. Much of it is man-made. In the 1970s, a dam enlarged the original lake to produce hydroelectric power. Today, that power is an important part of the country's electricity supply. The wind across the lake is often strong, which has also made it a popular place for windsurfing and kitesurfing.

Arenal also has an older neighbour. Close by stands Cerro Chato, a much older volcano that has long been dormant. Its crater holds a small green lake, surrounded by forest. The hike up is steep and often muddy, so it suits experienced walkers. Together, these two peaks show two stages in a volcano's life. One is young, sharp and still breathing. The other is old, softened by rain and trees.

The heat below the ground still shapes the area. Rainwater sinks deep into the earth, warms near hot rock, and rises again as springs. That is why the roads around the park are lined with hot spring resorts and natural pools. Nearby, the town of La Fortuna grew from a small farming village into the main base for visitors. Its name means fortune. Many people say it was lucky, because the 1968 eruption mostly spread toward the other side of the volcano.

Before you leave, stop for a moment. Look at the dark rock under your feet and the bright green leaves around it. Listen again for the howlers, the birds, the drip of water from the canopy. Then look up toward the cone, whether it is clear or wrapped in cloud. Everything around you — the forest, the lake, the warm springs, even the town — exists because of that mountain. And as you walk away, the volcano is still there, quiet for now, watching the valley it made.