Infdev is a mode still under development, which includes a map area said to be larger than the Earths area. It generates strange landforms, including floating islands and strange looking cliffs. If you die, you can respawn at your starting point, but you drop all of your items you were carrying and can pick them up if you go back to where you're body is. Other than that, it it somewhat like indev.
You must buy the game in order to play infdev.