The Well of Souls is a phenomenon seen on many worlds and believed to exist on every planet that can, has and will eventually breath life. It was first mentioned in SolZen: Wisdom of the Sun.
The well is a nexus of souls, its the place where the biosphere of a planet and the hidden world beyond the veil merge, entwine and embrace. When things die their spirits are often drawn to the nearest Well or phase through the ether. Though it is called a well, each well may not always take that form, in fact some appears as crystals or caves, or even trees.
Wells are places closely tied to the 'Hidden World' or Spirit Realm, as some would call it. It is believed that these places are traditions holding supernatural abilities begin (magic). Like many mystical things, their existence is not well received by the scientific minds of the universe.
A well is also a source of knowledge as it connects to the Memory of the Stars or Ahkashic Records, hence it is usually illegal to interfere with them, not the many know how...