The serious incident occurred in the Helsinki Terminal Control Area on 26.3.2008 at 14:39 UTC. An Ukraine International Airlines™ Boeing 737-300 airliner descended below its ATC clearance altitude. While being radar vectored to the Helsinki-Vantaa aerodrome the airliner was cleared to 2300 FT. However, it descended to 1400 FT, at its minimum, breaking the minimum terrain clearance with a telecommunications mast ahead of its track. The air traffic controller noticed that the airliner had deviated from its clearance altitude and ordered it to immediately return to 2300 FT.