Simplify the following text for a Grade 12 native English-speaking person.
Original: In 1876 the archaeologist Heinrich Schliemann discovered in Mycenae six graves , which he was confident belonged to kings and ancient Greek heroesâ '' Agamemnon , Cassandra , Evrimdon and their associates .
Simplified: In 1876 the archaeologist Heinrich Schliemann discovered in Mycenae six graves , which he believed were those of kings and ancient Greek heroesâ '' Agamemnon , Cassandra , Evrimdon and others .