Earth or Terraria, is a dimension outside of Aetheria. Prior to the crash, Earth was considered a myth to most Aetherians, as they didn't believe in it.
Earth has different physics than Aetheria. As stated by Mike, trees fall over when cut on Earth.
The world is thought to have been obliterated in a devastating nuclear exchange. The attackers seem to be from the East (Most likely from Asia or the Middle East).
Little is known about this world to the Aetherians. It most likely does not exist as it was destroyed by nuclear warfare, or is too irradiated and toxic to sustain life. Or some humans survived but are locked underground as stated by Kolb when he wasn't able to contact his parents.