Simplify the following text for a Grade 6 native English-speaking person.
Original: The arena substructure was partly excavated in 1810 â `` 1814 and 1874 and was fully exposed under Benito Mussolini in the 1930s .
Simplified: The underground area was partly dug out in 1810 â `` 1814 and 1874 . This digging was finished by Benito Mussolini in the 1930s .