Pelasht, originally called Phelsh't, was a location on Vulcan.
This region was the home of an ancient stronghold which was carved out of the rocks of the mountain itself. Internally, it possessed a large chamber called the Hall of Pelasht which was 500 meters high with a shaft to the outside. Its importance in Vulcan history was due to the fact that it was made to protect a source of natural water which sprang from the cool rocks. In times past, Pelasht was often a battleground where invading clans were lured into the grandspace where they were bombarded by heavy rocks that were dropped from the holes above. The interior was lit by lamps that were carved into the rocks.
When the Vulcan species first arose into existence as a tribal grouping of families, they huddled in small clans which sought the few water springs in order to continue a meagre living. One such tribe, however, developed a mutation in their eyes which gave them an inner eyelid which protected them from the sun. This allowed them to storm the fortress of Phelsh't in order to attain the rich spring of cascading water. After taking the region, the leader of the tribe began to use the area as their home and employed their inner eyelid mutation to form strategic alliances with other tribes.
By the modern era, Pelasht became partly the home of the Academy of Sciences. The old hall at Pelasht in the Academy was both a banquet hall and had a ceremonial purpose to it. The rest of the location was spread out towards the city where a series of buildings were noted for their beautiful appearance despite the modern construction used to build them. (TOS novel: Spock's World)