City of Shadows
The Dark — rare
Land
City of Shadows rulings:
2009-10-01: You can activate City of Shadows’s second ability while it has no storage counters on it, though it won’t add any mana to your mana pool in this case. You did not tap it for mana, though, in case any abilities (such as the ability of a Fertile Ground enchanting it) care about that.
2004-10-04: The storage counters aren’t removed when you activate the mana ability.