The Abbey of Santo Spirito al Morrone (locally Badia Morronese) was founded in the late 13th century by Pietro da Morrone and became the mother house of the Celestine order. The complex is enormous — much larger than its current state of partial restoration suggests — with multiple cloisters, a Baroque church, and outbuildings that once housed hundreds of monks.
The abbey was suppressed in the 19th century and used for a long time as a prison; the prison closed only in the late 20th century. Today the complex is gradually being restored and opened to the public, with the church and main cloister accessible. Look out for the cell of Pope Celestine V, preserved as a relic. The site is at the foot of Monte Morrone, with the hermitage of Sant'Onofrio visible on the cliffs above.