Simplify the following text for a Grade 13 native English-speaking person.
Original: Along with Lake Van and Lake Urmia , Sevan was considered one of the three great lakes of the historical Armenian Kingdom , collectively referred to as the Seas of Armenia ; it is the only one within the boundaries of today 's Republic of Armenia .
Simplified: Sevan was one of the three great lakes of the historical Armenian Kingdom , collectively referred to as the Seas of Armenia , and it is the only one within the boundaries of today 's Republic of Armenia .