On the evening of March 5, 1854, nine men associated with the Know-Nothing party snuck up to the base of the Washington Monument and made off with a rather hefty hunk of stone. The men carried the stone to a boat waiting on the tidal basin, smashed it into pieces and dumped it in the middle of the Potomac.
You may be curious as to why they (or we!) were interested in an old — and probably really heavy — rock. Where exactly did this stone come from and why was it such a big deal when it was stolen and destroyed? Maybe it was the fact that it came from the Pope... Just a guess.