The Underground Chambers are a set of chambers under Hogwarts. In Harry Potter and the Philosopher's Stone, Harry, Ron and Hermione enter these chambers to retrieve the Philosopher's Stone.
Location
Fluffy guarding the trapdoor
The Underground Chambers are located under the school. They can be accessed through a trapdoor on the Third Floor, in the Forbidden Corridor. This trapdoor is guarded by Fluffy, Hagrid's monstrous three-headed dog.
First Chamber
The first chamber is the chamber under the trapdoor. It has a sturdy net of Devil's Snare to catch whoever falls through the trapdoor. If the wizard in question does not quickly find a way to get out of the net, they will be strangled by the plant.
The First Chamber is not available on Pottermore.
Second Chamber
The Second Chamber, with the Winged Keys
The second chamber is a chamber full of winged keys. One of these keys needs to be caught in order to unlock the door to the next chamber. On Pottermore, the player has to find the Winged Key among many others and successfully capture it in order to unlock the next moment.
Third Chamber
The third chamber holds a fully-grown mountain troll. By the time Harry, Ron and Hermione get to this chamber, the troll has already been knocked out, so they can move on to the next chamber easily.
The Third Chamber is not available on Pottermore.
Fourth Chamber
The Chessboard Chamber
The fourth chamber is also known as the Chessboard Chamber. This chamber is a massive Wizard's Chess game, devised by Professor McGonagall. Ron, being the Wizard's Chess expert among the trio, takes charge and leads the group to the other side, although he has to sacrifice himself in the process.
The Chessboard Chamber is featured on Pottermore, but there is nothing to do or find in the moment.
The Chess Chamber is also now available on Pottermore on PlayStation Home. You can now play Wizards chess with other PlayStation Home users.
Fifth Chamber
The Potion Puzzle
The fifth chamber is also known as the Potion Puzzle. As soon as this room is entered, flames block the entrance and exit. The only way to get out is by finding out which of the six potions on the table will safely let the wizard pass through the flames.
In the book, Hermione figures out the riddle. On Pottermore, the player has to solve the clues hidden in the riddle and select two potions to move on: the potion to continue to the next chamber, and the potion to return to the previous chamber.
Sixth Chamber
The final chamber
The sixth chamber is the final chamber. It holds the Mirror of Erised, in which the Philosopher's Stone is hidden. On Pottermore, the player has to find the Stone before being able to move on to the next moment.
From the Story
"Fluffy, the monstrous, three-headed dog placed at Hogwarts to guard the Philosopher's Stone, stands on a trapdoor that leads to underground chambers beneath the school. The chambers all hold different protective enchantments, each set up by a different teacher and presenting a different type of challenge to the wizard who seeks to pass through them. Professor Sprout fills a room with the dangerous and immobilising Devil's Snare, Professor Flitwick charms hundreds of keys to mask the one that will open the door; Professor Quirrell brings in a troll, Professor McGonagall transfigures life-size chess pieces that must be defeated for a person to pass.
"Professors Snape and Dumbledore design the final two chambers. Snape presents a logic puzzle that, when solved, identifies the potion a wizard must drink to advance, with several of the incorrect choices resulting in certain death. Professor Dumbledore places the Mirror of Erised in the final chamber. It is charmed to give the Philosopher's Stone only to someone who wishes to acquire but not use it."