Complex sentence: About 1,700 years ago, the Hopewell people built Nanih Waiya, a great earthwork mound in central Mississippi, which is still considered sacred by the Choctaw.
Split into simpler sentences: About 1,700 years ago, the Hopewell people built Nanih Waiya, a great earthwork mound in central present-day Mississippi. It is still considered sacred by the Choctaw.