Locals Called It Just a Mountain — Then It Erupted Without Warning

For centuries before 1968, the people living at the foot of Arenal called it Cerro Arenal — Arenal Hill — because nothing in living memory suggested it was anything other than a quiet, forested mountain. On the morning of July 29, 1968, it erupted with no warning at all, hurling rocks weighing several tons more than a kilometer at extreme speed and burying three villages — Tabacón, Pueblo Nuevo, and San Luís — under lava, ash, and debris. Eighty-seven people died, some swept away trying to cross a stream that the eruption had turned into a torrent within minutes. The Las Coladas trail that visitors now walk across at the base of the volcano runs directly over that same hardened 1968 flow.

Lake Arenal hides an entire drowned town beneath its surface. When the dam was built in 1979, it flooded the original settlement of Arenal along with the nearby town of Tronadora, and in dry years when the water drops low enough, the old church steeple still breaks the surface near the shore. The dam that submerged those towns now generates a large share of the country's electricity — the lake framing the volcano views on the Day 2 shore drive is, underneath, a working power plant.