The Grutas de las Maravillas (Caves of Marvels) are really worth a visit.
Buy your ticket from the office opposite and you will soon be off on a guided tour through the caves by a Spanish speaking guide.
(don't worry, you don't need to be able to understand, you can still enjoy the spectacle)
In 1976, the Movie At the Earths Core was filmed here. The route is well laid out and there are many and varied 'rooms' or sections with huge Cathedral like, cavernous spaces and small lakes. There are safe, easy steps and paths that wind the route so you don't need to be super fit.
The highlight of the tour is toward the end in the bizarre 'Salon de los Desnudos'' where the shapes of the formations take on an incredible similarity to the human form (to the surprise or delight of first timers to the caves)
This part of the tour is highly amusing, yet at the same time - incredible and awesome.
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