Complex sentence: The mountain is densely covered by eucalyptus trees that were imported from Australia during the reign of Menelik II, and mostly planted during Emperor Haile Selassie's reign; thus it is sometimes referred to as the "lung of Addis Ababa".
Split into simpler sentences: The mountain is densely covered by eucalyptus trees that were imported from Australia during the reign of Menelik II, and mostly planted during Emperor Haile Selassie's reign. Thus, it is sometimes referred to as the "lung of Addis Ababa".