Landing with a Porpoise.

Notice Number: NOTC2675

When a bounced landing is improperly recovered, the airplane may land nose first  -- setting off a series of motions that imitate the jumps and dives of a porpoise. This problem is usually from improper airplane attitude at touchdown.
 
The corrective action for a porpoise is the same as for a bounce and depends on the severity of the oscillations. When it is very slight and there is no large change in the airplane’s pitch attitude, a follow-up landing may be executed by applying sufficient power to cushion the subsequent touchdown, and smoothly adjusting the pitch to the proper touchdown attitude.

When a porpoise is severe, the safest procedure is to EXECUTE A GO-AROUND IMMEDIATELY. In a severe porpoise, the airplane’s pitch oscillations can become progressively worse, until the airplane strikes the runway nose first with sufficient force to collapse the nose gear.

Do you want to know more? The Airplane Flying Handbook and other FAA manuals are available here.