Looking more closely, they identified burned bits of animal bones as well. (discovermagazine.com)
Bone soup, commonly referred to as bone broth, or soup stock, is a liquid made by boiling animal bones in water with a little bit of vinegar. (pearltrees.com)
You might have guessed it, it is exactly what you thought it was, it is broth made from animal bones. (thehealthytart.com)