Early records indicate that this inland sea spanned 300 miles north to south and twice that in width. The Core Sea was also part of the Nermanis Sea. This sea provided succor and trade between the many dwellings of a halfling civilization called the Nermanis. The largest settlement on this sea was Maurkac.
In 920, the waters of this great sea began to recede. At the time the cause was unknown, with many Nermaneans speculating that they had been forsaken by Avandra. The real cause ended up being the Mîmêk city-state Sigatharkin and their war with Runelderil. In the Onyx Enmity War (915 - 920), the Mîmêk brought down a large section of an enormous and high cavern called Zarag-Gûn. In this undertaking, the Mîmêk were goaded by the priests of Tarâk with him supposedly telling them that it would bring an end to the war and their enemies. The drow account of the episode says that the Mîmêk did not realize that the 500' high roof of this cavern also served as the floor of the Nermanis Sea. The Mîmêk account says that the engineers miscalculated the amount of black powder and the strength of the bedrock. When the demolition went off, a catastrophe ensued. The force of incoming water expanded the hole, drowning both armies, devastating their city-states and had wide-reaching effects. The loss of the sea eventually brought down Nermanis. The Nermaneans, heavily reliant on the fresh water and fish of this sea, experienced famine and a drought of apocalyptic proportions. The sea had long kept the otherwise arid region around it in equilibrium, with frequent rainfalls and cool breezes. The loss of this sea to the blackness below changed these conditions for centuries. Another affected by this were the kuo-toans of Gholompotl. They saw their city, once submerged near the sea's bottom, become exposed to the "dryness and blistering rays of the stars", forcing their city's abandonment.
Beneath, in the subterranean region Faelgor, the draining sea submerged Zarag-Gûn and parts of the Sigan-Kûn passage. It also created a wide and deep canyon called Avardâk. The Mîmêk city-state Sigatharkin was flooded then submerged for decades. The survivors fled north where they founded the settlement Nurum-Maluk. The drow of Runelderil suffered much worse, losing nearly twenty thousand people. The survivors, several thousand drow migrated northeast where they built a new capital.
Two centuries after the Nermanis food, the climate of central Cinazan stabilized, becoming more temperate, replacing the severe drought conditions experienced after the sea's disappearance. Today, the old seabed is a rich fertile basin supplying the grain requirements of the entire Khazarkar Empire and some.