The most widely known and accessed massive collection for the public is The 'Mixing Secrets' Free Multitrack Download Library
In a standard audio file, all instruments are blended into a single stereo track. A multitrack collection, however, provides the "stems"—individual recordings of the drums, bass, vocals, and guitars in isolation. By providing these raw components, this collection allows researchers and creators to look "under the hood" of a production. It has become the gold standard for training Artificial Intelligence in tasks like source separation (the ability for software to pull a vocal out of a finished song) and automatic transcription. Bridging Art and Science
Analog tape from the 1970s and 80s absorbs moisture. If you play a tape that hasn't been "baked," the oxide falls off the backing, destroying the recording forever.