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Ruinas Jesuíticas de la Santísima Trinidad del Paraná
Photo: via Wikimedia Commons, GFDL
It'll take one to two hours to see the whole area. Also features a small museum.
La Santísima Trinidad de Paraná, or the Most Holy Trinity of Paraná, is the name of a former Jesuit reduction in Paraguay. It is an example of one of the many Jesuit reductions, small colonies established by the missionaries in various locations in South America, such as Brazil, Argentina and Paraguay throughout the 17th and 18th century. These missions were built as self-contained societies that existed outside of regular Spanish colonial life that integrated indigenous populations with Christian faith.
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These ruins are well worth a visit and the 10 km to get there. Bring lunch and enjoy it in the old Plaza Mejor.