After three years and 70 episodes, the hit tween series “iCarly” has carefully kept one big mystery going in the story of the high-school-girl-turned-Web-star.

No, not where the heck are her parents — who famously have not been home since 2007.

But how come we have never seen that most treasured place of teen asylum — Carly’s bedroom?

“It was not by design,” says Dan Schneider, the creator of “iCarly,” as well as nearly a half-dozen other Nickelodeon tween hits, including “Drake & Josh” and “Victorious.”

“We just never needed it.”

Schneider estimates he gets four or five emails and tweets a day from fans asking about Carly’s room.

” ‘Why haven’t we shown it?’ ‘What does it look like?’ Stuff like that,” he says.

In an episode titled “iGot a Hot Room” airing next Friday, the first glimpse fans will get of Carly’s room will show it as a burned-out, smoking ruin.

Carly’s brother Spencer makes her a birthday present that bursts into flames — another running gag on the show — and burns down her room.

By the end of the episode, Spencer and friends rebuild the room into a kid’s dream pad — including a trampoline aimed at the bed that Carly uses every night to launch herself under the covers.

“Fact is,” says Schneider. “I want one of those for home.”