Underground Chambers

The Philosopher's Stone chambers were a series of rooms underneath Hogwarts School of Witchcraft and Wizardry that served as the defences surrounding the Philosopher's Stone. There were seven chambers in total that contained an obstacle or task that guarded the stone from being stolen.

First obstacle


The only known access way was from a trapdoor in the Third Floor Corridor, which was guarded by the three-headed dog, Fluffy. This obstacle was given by Hagrid. To get past this, one must "play a bit of music" and put Fluffy to sleep. Fluffy guards a trap door that leads to the next task. Harry plays a wooden flute to get past Fluffy and there was another instrument on the ground which was a harp, used by Professor Quirrell



Second obstacle
After going through the trapdoor, a long drop led to a chamber filled with Devil's Snare, planted by the Herbology teacher, Professor Sprout. The Devil's Snare was designed to ensnare and tangle up any intruders. If unfamiliar with the plant, one will be strangled to death. The way to get past the plant was to produce light. To pass the Snare, Hermione uses a spell that created a jet of Bluebell flames.

Third obstacle


A downward sloping passage led to another chamber filled with flying keys, charmed by Professor Flitwick. A large wooden door led to another chamber. The door would not open with the use of any charm such as Alohomora, one needed the correct key to get through. To pass, one must use the broomstick provided to catch the correct key. In the movie, once airborne, the other keys will try to attack the pursuer, but nothing to this effect occurs in the book.



Fourth obstacle
The next chamber is dark and resembled a graveyard, it contained a Giant Wizard's Chess set, enchanted by Professor McGonagall. To defeat the game, one had to play themselves across the board and checkmate the King according to the rules of Chess. One cannot simply walk across the board and pass, so an adequate knowledge in Wizard's Chess is needed to complete this task.



Fifth obstacle
The following chamber contained a Mountain Troll that needed to be defeated in order to pass. This chamber had a terrible smell to it and it was of great relief to Harry and Hermione that they didn't have to be in there too long. This was provided by Professor Quirrell. When Harry and Hermione got there, the troll was already knocked out thanks to Professor Quirrell.

Sixth obstacle


The next obstacle guarding the Stone was a logical riddle (Hermione said that some of the great wizards haven't got an ounce of logic) that one had to solve in order to progress, set by Professor Snape. Upon entering the chamber, the doorway through which one entered is blocked by a purple flame. The other entry way leading to the Philosopher's Stone, was blocked with black flame. A table with seven bottle of differing shapes sat in the middle of the room.

Each bottle contained a liquid, but the bottles were not labelled with their contents. Three were poison, two contained nettle wine, one contained a potion to safely traverse the black flame, and the last contained a potion that allowed passage back through the purple flames. The riddle, when properly worked out, revealed the contents of each bottle, and which one was safe to drink to allow onward progress:

"Danger lies before you, while safety lies behind, Two of us will help you, whichever you would find,  One among us seven will let you move ahead,  Another will transport the drinker back instead,  Two among our number hold only nettle wine,  Three of us are killers, waiting hidden in line.  Choose, unless you wish to stay here for evermore,  To help you in your choice, we give you these clues four:  First, however slyly the poison tries to hide  You will always find some on nettle wine's left side;  Second, different are those who stand at either end,  But if you would move onwards neither is your friend;  Third, as you see clearly, all are different size,  Neither dwarf nor giant holds death in their insides;  Fourth, the second left and the second on the right  Are twins once you taste them, though different at first sight." [src]

Based on Hermione's deductions, from left to right, the potions lineup is:
 * 1) Poison
 * 2) Nettle Wine
 * 3) Potion to move the drinker forward through black flames
 * 4) Poison
 * 5) Poison
 * 6) Nettle Wine
 * 7) Potion to move the drinker backwards through purple flames
 * Note: one of the poisons was Weedosoros.



Seventh obstacle
The final obstacle was the Mirror of Erised, the challenge was designed by Albus Dumbledore, and probably the most difficult. The only way to obtain the stone was to look into the mirror, see yourself having it, but not using it.

Most people would see themselves using it, such as turning things to gold, crafting Elixir of Life, or giving it to someone. This is what Quirrell saw. Harry however, was able to get the Stone because he only wanted to save it and not use it.

Behind the scenes

 * It is very curious as to why J. K. Rowling chose to have the number of obstacles as she did. Seven obstacles were in place to get to the Philosopher's Stone and seven different potions were available in Snape's logic riddle. Seven, as deduced by Arithmancer Bridget Wenlock, has magical properties to it. It is believed that the number seven is the most magical number.
 * The fifth and sixth obstacles were cut from the film. The two obstacles omitted being the challenges set up by Snape and Quirrell. This is ironic, as Snape was the one who was suspected to want to steal the stone, and Quirrell was the one who actually wanted to. This is also a bit strange because in the film Hagrid still says that Snape is one of the teachers protecting the stone.
 * At one point in the "Battle of Hogwarts" chapter in Deathly Hallows, Hermione asked Ron if he is a wizard or not. This might have been a reference to what was said in the Devil's Snare Chamber by Ron to Hermione.
 * There were a lot of differences in the Chambers between the books, films and video games. The following table shows some:

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Philosopher's Stone (video game)
 * Lego Harry Potter: Years 1-4