D&D 3.0: Tales from Mystara
Port Siers was a secundary port located on Port Siers Bay on Ierendi Island. In 786 the entire population – over 1,000 people – was killed in less than an hour by a cloud of burning ash that descended from Mount Kikapua. The port has since been abandoned.
Port Siers sits on top of a Nithian ruin, buried under dirt and volcanic rock. Persistent Makai legend claims there was once a volcano near it in the mountains. If this is the case, no one knows what happened to it, though SOMETHING produced the layer of volcanic rock between the ruins and the city atop it. The Peoples Temple here has a small collection of Nithian boats on display, all taken from the ruins. Fire creatures and undead seem to be fighting it out futilely for control of the ruins, in the accessible areas.
The less accessible areas of the city had become a dangerous place with buildings submerged in the sea water and a swamp area swallowing what once was the center of the port city. The remains of buildings and boats (and even animals and people), are covered in a grayish coat of dirt that is covered with algae or mud.