From p.30 of the ALttP manual: "There are eight magical tiles which are entrances to the Dark World that was sealed by the Wise Men long ago. Most of these are hidden under objects like boulders."

The manual doesn't say how they were created. My theory: In OoT the Master Sword guarded 7 of these tiles. When Link pulled the Master Sword, the tiles opened and it was easy for Ganondorf to find his way into the Sacred Realm. After my ending, though, the Master Sword is still guarding these tiles. Ganondorf stumbles across the eighth one by accident, and when Link moves the Master Sword to the Lost Woods, the other seven tiles are opened. This is why, in ALttP, people could get to the Dark World, but they couldn't get back.