PARQUE NACIONAL VOLCÁN POÁS (daily 8am–4pm; $10, $2 parking; t 2482-2165), 38km north of Alajuela, is home to one of the world’s most accessible active volcanoes, with a history of eruptions that goes back eleven million years. Poás’ last gigantic blowout was on January 25, 1910, when it dumped 640,000 tons of ash on the surrounding area, and from time to time you may find the volcano off-limits due to sulphurous gas emissions and other seismic activities – it was closed for a while following the Chinchona Earthquake in January 2009. It’s worth checking conditions with the park before you set off, but even if all’s well, you’ll still need to get to the volcano before the clouds roll in, which they invariably do at around 10am.

Though measuring just 65 square kilometres, Poás packs a punch: it’s a strange, otherworldly landscape, dotted with smoking fumaroles (steam vents) and tough ferns and trees valiantly surviving regular scaldings with sulphurous gases – the battle-scarred sombrilla de pobre, or poor man’s umbrella, looks the most woebegone. The volcano itself has blasted out three craters in its lifetime, and due to the more-or-less constant activity, the appearance of the main crater changes regularly – it’s currently 1500m wide and filled with milky turquoise water from which sulphurous gases waft and bubble (with a ph value of 0.8, this is reputably the most acidic lake on earth). Although it’s an impressive sight, you only need about fifteen minutes’ viewing and picture-snapping; otherwise, you can take one of the short trails that lead off the main route to the crater.

The park’s visitor centre shows film of the volcano – handy if the real thing is covered by cloud – and has a couple of displays explaining the science behind it; there’s also a simple snack shop, but you’re probably better off packing a picnic or grabbing lunch at one of the nearby restaurants (see Watching wildlife at Volcán Poás).