Speech is distorted in pressurized helium-air. Many aspects, except for upward shifts of formant and pitch in the helium speech, however, have not been known yet. A helium speech database system including more than 5 000 words and messages of divers was developed. The vibration of the cheek, the neck, and the nose were also recorded for some utterances. They were taken at the simulation dives up to the depth of 300 m at JAMSTEC since 1982. The helium speech and the normal speech were compared in terms of (1) duration of phonemes, and (2) pitch frequencies and formant trajectories in two different hearing/speaking conditions: the first condition is a case when a diver spoke as he was listening to his own unscrambled speech, and in the other condition the diver was not hearing his unscrambled speech. As a result, the shorter duration of word-initial consonants, higher pitch by psychological stress, and rough movement of formant trajectories were observed in addition to the well-known formant and pitch frequency shifts. As a special property of helium speech, it is newly discovered that there exists a quasiformant below the normal first formant, and that it may be caused by the sound from the nostrils.