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.