The "X" is double sharp.
The "box" is a Breve (also called Double-Whole note - so it would get 8 counts in 4/4 meter, though you'd usually see this in something like 4/2 meter where it would get the whole measure's worth of time).
It's one ledger line and a space below the bass clef staff.
So it's a D Double-Sharp (which sounds like E) for 8 quarter note counts.
Do I get a prize :-)
Steve