Benthic Explorers checks the effects of all mana-producing abilities of the land untapped to pay its cost, but it doesn't check the costs of those abilities. For example, Vivid Crag has the ability ", Remove a charge counter from Vivid Crag: Add one mana of any color." If you untap a tapped Vivid Crag an opponent controls while paying for Benthic Explorers's ability, you can add one mana of any color. It doesn't matter whether that Vivid Crag has a charge counter on it.