In Santa Maria Delle Visitazione, in the Assassin's Tomb, you'll come across a large
flooded room just before one of the Armor of Altair seals. Pull the lever and peer 
into the water. Wait a minute or so and you'll see a shadow below. Pull the lever 
again and peer into the murky depths once more and a tentacle will reach out to grab
(and miss) Ezio.