City Under the Sea - Ernie Moulton - Bog

The level of an inland sea, the Mistal Sea, is sinking several inches a day. Mista is asked to try to find out the cause. When he starts investigating, he is presented with a tale of a city buried under the sea. There are clues to its location, and he organizes a band of adventurers and searches for the city, at the same time finding a cause for the sea level dropping. He is opposed by a mysterious person who wants the possible lost treasures in that city for himself. This person fights Mista and his friends at every turn. In the process of finding the city, the band forms bonds that will last for years.