In any maze, you will discover Eight Trigrams Furnace on an early floor. Interacting with it gives the options to "Kick it away" or "Go inside". Kicking it immediately awards three Elixir of Life. If you go inside, you instead get a +15% chance to find the key each floor for the current run. You can still kick the furnace afterward, but will not receive the elixers.
If you kicked the furnace at all, you will find a burning floor after 20F and encounter Flaming Mountain and Earth God; you take 3 damage each round on this floor. Earth God explains why this floor is so hot, but nothing else. Earth God can be brainwashed by Athena.
After finding Flaming Mountain, it and the burning effect will still be present if the floor is repeated on that run (such as after casting Portal of Earth).