Flame pit

A Flame pit is an Oubliette trap. It takes 77 Construction to build. A Flame pit hits constant 1-3s while any players are in the trap. A cage must first be built in order to build the flame pit but the cage can be removed after the flame pit is added. This glitch has been fixed now.

It costs 125,000 coins AND 20 tinderboxes in your inventory to build (they can't be noted) and building one grants 357 experience each, but no firemaking experience, as suggested.

Note: A crystal saw does work when making this, and so the minimum construction level needed to build a flame pit is 68 construction (Evil Stew + Crystal Saw) providing you already have an oubliette.

Flame pits are popular as a way of killing players in a house, letting them recharge prayer, summoning, hitpoints, and cure poison at the same time.

Flame pit glitch
The Flame pit glitch was a construction bug that allowed players to stand in the flame pit in an oubliette of a Player Owned House without getting hurt. To perform it, the house owner must first turn off the PvP mode of their dungeon using the lever in their throne room and then drop a player into the oubliette below. This causes the player dropped to not get hurt by the flames. The owner may also turn PvP on when the player is in the flame pit, and drop other players. These players will get hurt by the flame pit normally, and the player immune to the flames may also attack the other players. This glitch has been fixed now.