Unlike we previously thought, this game was actuallyreleased on the german disk magazine "Input 64" in the year 1986 as "Die versunkene Stadt" (sunken city).
Gamebase64 has the game here:
http://www.gamebase64.com/game.php?id=8381&d=18&h=0
This entry as a result will be removed in the future.
Case closed!
Contributions: Endurion