The Sea of Vastness is a sea located even beyond Beyond the Beyond. The River of Wind goes across it and the Middle Kingdoms are found across this sea. Along with Beyond the Beyond, this was the only location that also appeared in the Wolves of the Beyond series that also appeared in Guardians of Ga'Hoole. Apparently, there is an ice bridge across this sea that leads to the Distant Blue. It is also referred to as the Unnamed Sea and the Frozen Sea.
Soren and the Chaw of Chaws fly across the sea on the way to the kingdoms.
In this book, the sea plays a bigger role. Faolan and his friends cross an ice bridge across the sea. It is also revealed to be the only area in the series that has narwhales.