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.
They are believed to have been built thousands of years. These years are ago by an alien species known as the Heechee. Until an explorer discovers a Heechee ship in one of the tunnels. This is intact and operational. This was what Little is known about them.