Aroania, Mountain peak in Kalavryta Municipality, Greece

Aroania, also called Chelmos, is a mountain range in southeastern Achaea near the Corinthia border, whose highest summit, Psili Koryfi, reaches 2,355 m. Between roughly 800 and 1,800 m the slopes are covered with pine forest, while the higher ground gives way to grassland and bare rock. Its other named peaks include Neraidorachi, Aetorachi, Kokkinovrachos, Gardiki, Avgo and Nisi.

In antiquity the range was called Aroania. The later name Chelmos is explained either as South Slavic chlmo, 'summit', or as Albanian helm, 'poison', since the Styx, whose water the ancients held poisonous, rose on the mountain. Since 2009 the massif and the Vouraikos gorge form a national park.

The modern name Helmos is debated: one view derives it from a South Slavic word for hill, another from the Albanian helm, 'poison', because the Styx sprang from the mountain and its water was held to be poisonous. The Styx was one of the rivers of Hades, and the nymph Thetis bathed the infant Achilles in it.

The best time to visit for hiking is from May to October when trails are free of snow and conditions are safe. Several marked paths start from Kalavryta and lead up the mountain, requiring proper gear and decent fitness.

The Chelmos Observatory, home to the 2.3 m Aristarchos telescope, stands near the summit ridge rather than on the highest peak. High on the mountain, meltwater forms the seasonal alpine lake Mavrolimni at about 2,050 m, which shrinks as summer advances. The Chelmos blue butterfly, found only between 1,100 and 1,800 m, was first described from this mountain.