While improved defense is certainly nice, it doesn't do anything for mobility, so it's time to keep going. Using the Ocarina to reach a high ledge in the desert and the Powerful Gloves to lift a rock, Link can reach the Dark World swamp where the sixth labyrinth is. The swamp is predictably swampy, and it's pouring rain. There's no apparent entrance, but there is a platform in the middle of the swamp with a symbol that looks a lot like a medal engraved with a lightning bolt. Link tries standing there and using the Ether Magic. Sure enough, this stops the rain, and the entrance rises out of the water. The labyrinth seems rather goopier than the previous ones, but isn't all that much different, except for the (puny) wizards that appear out of nowhere and fire at Link. It's also probably the most maze-like labyrinth, with at least four more keys (and a locked door for each) than are actually required to complete it.
While searching for the big key, Link finds a telepathic tile:
４つのあかりをともせば
すすむ道が開けるじゃろう。
If you light the four torches,
the way forward will open.
the elder! You must set fire
to four torches to open
the way forward.
The four torches are in two different rooms and are thus fairly easy to overlook. Lighting them slides back the wall in the room with the tile, revealing a pit that leads to the big key. Now Link can open the big chest, after using the Hookshot and dashing to reach it before the floor collapses.
うまく使えば、とてもやくに
立つ、ふしぎなつえだ！
It's a mysterious staff that will really
come in handy if I use it well!
It will be very helpful if you
make proper use of it!
What a mysterious cane!
Another candidate for the most useless description ever...
When Link swings the staff, it makes a block appear, which is mainly useful for holding down buttons in the floor. Hitting the block with the staff makes it explode into fragments that fly off and damage enemies. Any use consumes Link's magic power. Predictably, he needs the blocks to hold down buttons so he can advance.
Eventually, Link reaches the boss, a giant eyeball sitting in slime and surrounded by smaller eyeballs. The smaller eyeballs hop out and attack while the giant one stays where it is and periodically shoots lightning down the middle of the room. The best strategy seems to be standing in a corner swinging madly to keep the smaller eyeballs at bay, until they die. Once only four small eyeballs (which don't do anything for the whole battle) remain, the giant one hops out and attacks Link. The remaining challenge is nonexistent. Explosion, the other little ones die too, heart container, crystal.
魔族の手から、のがれる事が
できました。ありがとう…。
ガノンが私達を集めたのは、
ガノンの力では７賢者の封印を
やぶれなかったからなの。
そこで、司祭アグニムを使って
賢者の力をうけつぐ娘７人を
闇世界におくりこませたの。
７人の力をりようして、封印を
やぶったあと、ようがすんだ
私達はクリスタルに封じられ、
魔物達にあたえられたわ。ただ
あなたの事を知らなかったのは
ガノンのミスだったわね。
さあ、ゼルダ姫がカメイワで
あなたをまっています。
急いで行ってあげて下さい。
勇者の行く道がトライフォースイ
へと、みちびかれますように。
the fiend's clutches because
of you. Thank you...
Ganon gathered us because he
was unable to break the Seal of the
Seven Sages with his power alone.
So he used the priest Agahnim to have
the seven maidens who inherit the power
of the Sages sent into the Dark World.
After he made use of our power
to break the seal, we were shut up
in crystals and given to monsters
since he had no further use for
us. Not knowing about you was
Ganon's only mistake.
Now, Princess Zelda awaits
you in Turtle Rock.
Please hurry and go to her.
May the way the hero travels
be guided unto the Triforce.
escape from the clutches of
the evil monsters. Thank you!
Ganon captured us because he
couldn't break the seal of the
wise men with his power alone.
And then, using the wizard
Agahnim as his pawn, he drew us
to the Dark World.
After cracking the seal with
our powers, he sealed us inside
of these crystals.
He then gave us to his loyal
monsters. But Ganon didn't
plan on your getting this far.
Now, Princess Zelda is waiting
for you inside of Turtle Rock.
Please hurry!
May the way of the Hero lead
to the Triforce.
By Ganon not knowing about Link, I think she means that he didn't realize Link's significance, not that he didn't know Link existed (though that's also possible).