Complex sentence: They are believed to have been built thousands of years ago by an alien species known as the Heechee, but little else is known about them until an explorer discovers a Heechee ship, intact and operational, in one of the tunnels.
Split into simpler sentences: They are believed to have been built thousands of years ago by an alien species known as the Heechee, but little else is known about them. That is, until an explorer discovers a Heechee ship, intact and operational, in one of the tunnels.