Untapping a tapped land an opponent controls is part of the cost of Benthic Explorers's ability. If no opponent controls a tapped land, the ability can't be activated.

The types of mana are white, blue, black, red, green, and colorless.

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 "{T}, 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.

When determining what types of mana an opponent's land could produce, take into account any applicable replacement effects that would apply to that land's mana abilities (such as Contamination's effect, for example). If there is more than one, consider them in any possible order.

Benthic Explorers doesn't care about any restrictions or riders an opponent's land (such as Ancient Ziggurat or Hall of the Bandit Lord) puts on the mana it produces. It just cares about types of mana.

This ability is not targeted any more.