The Cave

The Horcrux Cave was the cave where Albus Dumbledore and Harry Potter traveled to in the spring of 1997, in search of Salazar Slytherin's Locket which was one of Lord Voldemort's Horcruxes. It was a seaside cave located somewhere in Britain.

Early History
Before he became Lord Voldemort, Tom Marvolo Riddle once visited this cave, with his orphanage. During the trip he had terrorized fellow orphans Amy Benson and Dennis Bishop in this cave.

Horcrux hiding place
Voldemort later decided to use this cave as a location for one of his Horcruxes (Salazar Slytherin's Locket). Upon storing the locket in the cave, Voldemort set up a large number of complex enchantments, designed to keep others out but still allow him access to the Horcrux. Despite all of these defenses, Harry and Dumbledore successfully retrieved a fake version of Slytherin’s locket planted by Regulus Black and the house-elf Kreacher who also passed the defenses, and switched the real locket with the fake one, though Regulus fell to the Inferi Lake before he could escape and became one of the Inferi of the lake. It is suggested that this cave already had at least some of these defenses as Voldemort had to have Kreacher drink the potion before he could place the Locket in the basin although that could just be a test of its effectivness. If that's so then the cave is likely some kind of ancient Dark Magic stronghold that had been abandoned until Voldemort re-discovered it.
 * He placed a spell on the cave so that it was impossible to Apparate inside it (though ineffective against house-elf magic).
 * A smaller entrance was made - a door which required a blood sacrifice to open. According to Dumbledore, this was due to a common misconception by Voldemort that physical pain would weaken a person entering the chamber.
 * An Inferi-infested lake separated the entrance of the cave from the Horcrux. Any attempt to use a Summoning Charm to get the Horcrux would cause an Inferius to leap out of the water.
 * The Horcrux was placed on an island in the middle of the lake. To get there, one had to find an invisible boat anchored to the cavern wall. The boat was enchanted so that only one wizard of age could be transported in it.
 * The Horcrux itself was in a basin filled with a potion that could not be touched, vanished, transfigured, or charmed; the only way to dispose of it was to drink it. Once drunk, though, it caused the drinker to see 'terrible things', experience almost unbearable burning in one's insides, and induced dehydration. If the drinker still had strength left after consuming the potion, he would be unable to drink water that did not come from the lake; Upon disturbing the water an army of Inferi would rise from the lake and attempt to kill him.