Ko Tapu is a limestone rock about 20 metres (66 ft) tall with the diameter increasing from about 4 metres (13 ft) near the water level to about 8 metres (26 ft) at the top. It lies about 40 metres (130 ft) to the west from the northern part of Khao Phing Kan.
A local legend explains the formation of Ko Tapu island as follows. Once upon a time, there lived a fisherman who used to bring home much fish every time he went to the sea. However, one day he could not catch any fish despite tedious attempts and only picked up a nail with his net. He kept throwing the nail back to the sea and catching it again. Furious, he took his sword and cut the nail in halves, using all his power. Upon impact, one half of the nail jumped up and speared into the sea forming Ko Tapu.
A scientific version of the Ko Tapu formation says that in the Permian period, the area was a barrier reef. Then, upon tectonic movements, it ruptured, and its parts were dispersed over the area and flooded by the rising ocean. Wind, waves, water currents and tides gradually eroded the islands thus formed, sometimes producing peculiar shapes, such as Ko Tapu. Tide-related erosion is visible at the bottom of the rock.
source and more information: http://en.wikipedia.org/wiki/Khao_Phing_Kan