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Catacombs of Milos, one of the earliest Christian cemeteries in the world.
The massive Catacombs of Paris, a whole underground city
The Catacombs of Rome, many impressive underground momuments
Catacombs of Odessa
St Paul's and St Agatha's Catacombs, Malta
Basilica Cistern, Justinian
The Underground City of Naours
Salina Turda, Romania
Nekromanteion of Acheron. Not much more than a hole but it's ancient
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