Seas

Perhaps the most wondrous of the Underdark’s features are the vast, nighted seas of the deep earth. Seas are saltwater bodies, not fresh, and most of them are found in the Lowerdark, though Underdark seas also occur at higher spots beneath Tellene’s surface oceans. While air-filled cave systems may extend for dozens or even hundreds of miles beneath the oceans above, or system of air locked siphons of extraordinary size, these features are exceptional. Most caves beneath large bodies of saltwater are simply subterranean extensions of surface oceans.

Seas tend to form in the largest of vaults, occupying caverns large enough to be miniature worlds in their own right. Like the lakes, seas offer some of the best roads in the Underdark, and many are heavily traveled.