Their roots lie in the heavy rock of 70s groups like Deep Purple. The music they played--heavy metal mixed with punk attitude--became its own genre: thrash. Their bassist died and they survived to became the biggest-selling band in the world. As grunge threatened to overtake them, they reinvented themselves. Then their singer went into rehab and they almost fell apart. They are Metallica, the most influential heavy metal band of the last thirty years.