From the Introduction:
This adventure is designed for a party of 6 to 9 characters from 4th to 8th levels of experience each. It is meant to be used with the rules system found in the Basic and Expert D&D books edited by TOM MOLDVAY, DAVE COOK & STEVE MARSH, from the original game created by GARY GYGAX and DAVE ARNESON. A party that intends to explore the setting of this adventure should include at least one cleric, thief and magic-user of 5th to 8th level, and several fighting men, including dwarves, elves and hobbits.
...Note that this product does not relate in any way to Judges Guild world of High Fantasy setting
This scenario is an unofficial extension/sequel to X1: The Isle of Dread. The scenario is set in Mystara and introduces a sunken city for the adventurers to explore.
Myths and legends of a city beneath the waves are common gossip among seafaring folk that dwell along the coastlines of the great kingdoms. Sailors often talk of seeing mermaids far out at sea, weeks away from the sight of land. Could such creatures truly exist? Is there such a place where beings have erected an underground city? Only the most hardy and inquisitive adventurers are likely to investigate such a thing.