Complex sentence: The crater is a multiple-ring structure about across, with its diameter inner ring its most prominent feature; it contains a diameter annular lake, the Manicouagan Reservoir, surrounding an inner island plateau, René-Levasseur Island.
Split into simpler sentences: The crater is a multiple-ring structure about across, with its diameter inner ring its most prominent feature. It also contains a diameter annular lake--the Manicouagan Reservoir--surrounding an inner island plateau called René-Levasseur Island.