From the introduction: Welcome back intrepid Underdark adventurers! Journey Through the Center of the Underdark 2 - The Darklake Strikes Back contains several more encounters meant to flesh out your days spent traversing the underdark. This time we're on the vast and varied waters of the Darklake, where the Kua Toa are Kua Toa and the fish are frightened. ... Readers unfamiliar with Out of the Abyss or the Underdark of the Forgotten Realms are likely unaware of the existence of the Darklake, so an explanation is in order. The Darklake is the name given to a massive interconnected underground body of water hundreds of miles across that flows through all manner of Underdark tunnels. Gargantuan vaults of unfathomable size and depth, narrow maze like tunnels and broad water filled passages are some examples of the varied chambers in which this body of water resides.