Considering that Earth has been around for 4.54 billion years, and the Universe for 13.8 billion years, I think it is safe to say that NATURE offers the largest collection of content in the Public Domain...by FAR!
Also in the mix is the fact that the Copyright Office will refuse to register a copyright claim if it determines that a human being did not create the work. So unless some human that we are unaware of created the Universe and all that is in it, we are on solid ground for Nature easily being the most massive source for content NOT protected by copyright!
Let me provide you with some crazy, mind-blowing examples:
• Species of Trees: 60,065
• Species of Plants: 391,000
• Species of Vertebrate Animals: 80,500
• Species of Invertebrate Animals: 6,755,830
• Types of Minerals: 5,400
• Types of Elements: 118
• Types of Weather: 10
• Types of Snowflakes: 35
• Types of Climate Zones: 14
• Number of Colors: 1.07 billion
• Number of Sounds: ??
Now, factor in all the variations and combinations...yeah, it's a lot.
And then there are all the astronomical objects. You get the picture!