Why does my excel document have 960,000 empty rows?

Solution 1:

Microsoft has an excellent support document called How to reset the last cell in Excel.

From that document:

The most common cause of the last cell being set outside the worksheet range that is currently in use is excessive formatting. When you format whole rows and columns, some types of formatting can cause the last cell to be set to a cell far below or to the right of the actual range that is in use.

Based on the comments below your question, it certainly seems like the border format that is applied to all rows is the culprit.

I've used the code from the add-in provided from the link above to reduce file sizes from multiple megabytes to a few hundred k.

Solution 2:

Also check to see if there are any cells with comments where the comment box has been dragged down far away from the cell. You can have a cell in the second row of a spreadsheet with a comment whose comment box is near row 7000 and that will force excel to treat row 7000 as the spreadsheet's last row.