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A portal to a secret trove of treasure
"'Stop thief! Stop thief!' There is a magic in the sound. The tradesman
 leaves his counter, and the car-man his waggon; the butcher throws down
 his tray; the baker his basket; the milkman his pail; the errand-boy
 his parcels; the school-boy his marbles; the paviour his pickaxe; the
 child his battledore. Away they run, pell-mell, helter-skelter, slap-dash:
 tearing, yelling, screaming, knocking down the passengers as they turn
 the corners, rousing up the dogs, and astonishing the fowls: and streets,
 squares, and courts, re-echo with the sound."
    -Charles Dickens, "Oliver Twist"

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Trees
While the dungeon is filled with dim magic light, in most places it is not bright enough to sustain any larger plants.  It is uneven, though, and there are spots where, with the grace of Feawn, trees as big as those on the surface can grow underground.

A portal to a secret trove of treasure
"'Stop thief! Stop thief!' There is a magic in the sound. The tradesman
 leaves his counter, and the car-man his waggon; the butcher throws down
 his tray; the baker his basket; the milkman his pail; the errand-boy
 his parcels; the school-boy his marbles; the paviour his pickaxe; the
 child his battledore. Away they run, pell-mell, helter-skelter, slap-dash:
 tearing, yelling, screaming, knocking down the passengers as they turn
 the corners, rousing up the dogs, and astonishing the fowls: and streets,
 squares, and courts, re-echo with the sound."
    -Charles Dickens, "Oliver Twist"
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Trees
"Only YOU can prevent forest fires!"
    -Smokey the Bear