Children of Mana

When you've nearly beaten the game, equip an elemental spirit and speak to that same spirit in the Mana Village. The spirit should send you on a quest to meet the Benevodon of the corresponding element in another dungeon, and you'll obtain a special 'lex' item for completing the quest. Repeat this process for the other 7 elemental spirits and once you've gotten all 8 'lex' items, proceed through the game as normal until you beat it. When the game returns you to the Mana Village after the ending, the Mana Tree mural should be glowing. Approach it and you'll be transported to a secret bonus portion of the final dungeon, complete with new enemies.