Koutouki Cave, about 2 million years in the making by the karst phenomenon (chemical deposition of calcium carbonate through dripping water) on the eastern side of Mt. Hymettus at 510m asl, was accidentally discovered in 1926 when a goat disappeared through a crack and started bleating. The lucky goat survived what today would have been a 38 metre fall, as it landed softly on a cushion of soil and branches that had also fallen through. Almost thirty years passed until a full investigation and mapping was carried out by Ioannis and Anna Petrocheilos, the couple who also pioneered speleology in Greece, in the 1950s, while a 17m tunnel was created in the 1960s to enable visitation. The cave has a 60 x 60 m shape but the 350 metre circuit feels like a labyrinth thanks to 55 large columns and countless stalactites and stalagmites. It used to be a home to bats and it is still shelters endangered cave crickets (Dolichopoda petrochilosi). A new genus of cave spider (Sulcia kratochvil) was also discovered here in 1971, but it is unknown if it has survived tourism. Unfortunately there is no public transport to the site, so it is either private transport or a 75 minute (4 km) steep walk from central Paiania, the nearest town. The cave is open daily, with 30-minute tours every hour, between 08:30 and 14:30. 

A much-needed renovation in 2016 reversed years of neglect, eradicating the invasive greenery (Malattia Verde) that had spread across the cave walls, as in many other visited caves, due to improper warm lighting and the door being left open. Today, well-maintained walkways and handrails guide tourists through the cave, though taller and younger visitors should move with care. Regrettably there is no disabled access. 

As a bonus, on the way to the cave you can enjoy breathtaking vistas of eastern and southern Attica. Mt. Hymettus boasts two dozen caves and gullies, but Koutouki is certainly the most impressive.