The title of this article is conjectural. Although it is based on canonical information, the actual name is conjecture and may be supplanted at any time by additional information released from canonical sources. If this occurs, please move this page to the appropriate title.
"The greenish light seemed to be growing larger at last, and within minutes, the boat had come to a halt, bumping gently into something Harry could not see at first, but when he raised his illuminated wand he saw that they had reached a small island of smooth rock in the centre of the lake...The island was no larger than Dumbledore's office: an expanse of flat dark stone on which stood nothing but the source of the greenish light, which looked much brighter up close. Harry squinted at it; at first he thought it was a lamp of some kind, but then he saw that the light was coming from a stone basion rather like the Pensieve, which was set on top of a pedestal"
The island itself appears to be made of smooth rock, which made it difficult for Albus Dumbledore and Harry Potter to move about when they were searching for Lord Voldemort's Horcrux and battling the Inferi. In the middle of the island is a basin full of an emerald green potion, which must be drunk in order to get the Horcrux at the bottom. The potion would cause nightmarish delusions and extreme thirst, which would force the intruder to drink from the lake, as any other water produced would vanish, thus activating the Inferi defence.