This is one of my favourite places in Lisbon. The site itself, alone, is worth seeing for its beauty and poignancy: a large church nave open to the sky except for its skeletal Gothic arches. More than anywhere else, it is possible to imagine how terrible the 1755 earthquake must have been.
The church serves as a museum: the open nave is lined with various stone sculptures from monasteries and churches across Portugal, many of which are quite exquisite. The apse and chambers at the back contain various archaeological exhibits ranging from prehistoric artifacts to an outstanding royal medieval tomb and even some Peruvian mummies!
Unless you have less than two days in Lisbon and are spending one in Belém, you should definitely take the time to come here. It's right next to the Santa Justa elevator (from which there are also fantastic views over the city, opposite the Castelo São Jorge).
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