Take a look at this picture:
I think most of use agree that the "big silvery" thing on the board (red) is the famous 16 MHz crystal of the Arduino. That's what I've believed until very recently.
I was bored so I looked at my Arduino Uno, and saw that it has what seems like another crystal (yellow). It looks like the one that is on the Pro Mini. It's also located near the XTAL pins. After I inspected the schematic a second time, it also appears that there are two crystals on the board.
Which one out of those two is actually connected to the XTAL pins and why are there two?