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, 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 the waters to the blackness of the Underdark changed these conditions for centuries. The kuo-toans of Gholompotl abandoned their city when the Nermanis Sea drained away. Their settlement was found many years later and became part of Nirzanâth. The Mîmêk city-state Sigatharkin was flooded by the draining sea and then washed away. About ten thousand Mîmêk went north and formed colonies that would become Zandûl-Khâl. The drow of Runelderil suffered much worse, losing twenty thousand people. Several thousand drow migrated northeast and would later become Nîndûnturgac. Much of the Nermanis's waters flowed through the Sigan-Kûn passage. The onrushing waters of the Nermanis Sea bored this out and left a greatly expanded area.
In the early part of the Second Epoch, the climate of the region had changed to be more temperate than the severe drought conditions experienced after the draining of the sea.
The Nermanis Sea is gone, lost to the Underdark, with it went three civilizations. The area that once held the fresh waters of this sea is a fertile basin. The soil is rich in nutrients and results in rich harvest of grains. The grains supply the food requirements of the entire Khazarkar Empire and result in massive trade surpluses.
When the Nermanis Sea drained into the Underdark it created a wide and deep canyon called Avardâk. Water still drains into this canyon, but now its just rainwater and a few streams from upland. Explorers can enter the Underdark realm of Faeglor by way of Avardâk.