Iron Butterfly's debut album "Heavy" from 1968 is a powerful, organ-influenced work of psychedelic rock that laid the foundation for heavy metal. With dense guitar riffs, swirling keyboard sounds and Doug Ingle's haunting vocals, the album conveys a raw, atmospheric atmosphere throughout. Tracks like "Possession" and "Unconscious Power" show a band boldly experimenting with weight and distortion, creating a sound darker than most of their psychedelic counterparts.