Unremovable Double Underline

My document has a line going across the page that looks like a double
underline. It cannot be selected or deleted and is not caused by text
or paragraph formating as far as I can tell. The line can be moved up/
down by dragging.

I believe it was created automatically after I typed a few dashes
(---) followed by a CR. In case it helps, I tried this again
elsewhere in the doc and the dashes were converted to a SINGLE
horizontal line. Undo read: "Undo AutoFormat"

What is this? How do I kill it?