Underground Chambers

The Underground Chambers were a series of rooms underneath Hogwarts School of Witchcraft and Wizardry that served as the defences surrounding the Philosopher's Stone in the 1991-1992 school year. 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 set by Rubeus Hagrid. To get past this, one had to "play a bit of music" and put Fluffy to sleep. Fluffy guarded a trap door that leads to the next obstacle. Professor Quirrell most probably enchanted a harp to put Fluffy to sleep while Harry played a wooden flute.

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 entangle and suffocate any intruders, especially quickly the more the intruder attempts to escape. As the Snare is sensitive to heat and light, Hermione used a charm that created a jet of Bluebell flames to free Harry and Ron.

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 had to use the broomstick provided to catch the correct key. However, once airborne, the other keys will try and attack the pursuer.



Fourth obstacle
The next chamber was dark and resembled a graveyard, it contained a Human Chess Game, 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 of Wizard's Chess was 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 set by Professor Snape. Upon entering the chamber, the doorway through which one entered was 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 therefore 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) Weedosoros
 * 6) Nettle wine
 * 7) Potion to move the drinker backwards through purple flames

As Professor Quirrell presumably travelled through the chamber to reach the Mirror of Erised, he must have had to take the Black Fire Potion himself, and, as there was only enough potion for one person in the bottle, that would have used it all up. However, he may have taken some of the potion with him when he entered the chambers, and drunk that instead. Another possibility is that the bottles had had a Refilling Charm on them.



Seventh obstacle
The final obstacle was the Mirror of Erised, designed and set by Albus Dumbledore. 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

 * The prevalence of the number seven is not coincidental; 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. It is also, of course, the total number of books in the series, with Rowling firmly sticking to that number.
 * In the film adaption of Harry Potter and the Philosopher's Stone an Undetectable Extension Charm were possibly put at the Winged Key Chamber, as it was taller than the Devil's Snare Chamber and Fluffy Chamber together, and it could not be seen from the outside of the Hogwarts Castle.
 * The fifth and sixth obstacles were cut from the film. This is curious, 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. However, in the film, when the Trio unlocks the door, a bit of the Troll Chamber can be seen, and the Potion Riddle appears on Disc 2. It is possible that the film makers just cut from the Winged Keys Chamber to the Mirror of Erised Chamber.
 * At one point in the "Battle of Hogwarts" chapter in Deathly Hallows, Hermione asked Ron are you a wizard or not?. This might have been a reference to what was said in the Devil's Snare Chamber by Ron to Hermione, are you a witch or not?.
 * There were a lot of differences in the Chambers between the books, films and video games, shown in the following table:

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