Secret Bonus Dungeon
     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.