In the assassin's tomb in Santa Maria Delle Visitazione, before the room 
where
you get one of the seals required for the Armour of Altair, is a large room
filled with water. This room has an obstacle course. Activate the lever 
before
going through the course, then stand on the edge of the floor, and look at 
the
water. Remain idle until a large shape swims across the water. Use the lever
again after the timer resets, and stand at same location. After awhile, a 
giant
tentacle will shoot out of the water that Ezio dodges at the last second.