The Burrow Beast is unlocked through doing the final Arkvoodle mission in Takoshima (thus getting you the trophy for that too). The Pox-Mart upgrades for this weapon require 35 Furotech Cells to complete. This weapon is a bit overpowered and, at the same time, useless, as it launches a beacon out and the beast breaches the surface to eat all enemies around it (so it's pointless if you have to move about constantly).