Complex sentence: The Sanctuary of Poseidon has been excavated by Swedish archaeologists, one field season in 1894 considered to be the first Swedish archaeological campaign in Greece, and continued excavations from 1997 and onwards in collaboration with the Greek National Heritage Board.
Split into simpler sentences: The Sanctuary of Poseidon has been excavated by Swedish archaeologists. Swedes have continued excavations from 1997 in collaboration with the Greek National Heritage Board.