Trying to find a reasonable mana source in blue. Tapping down opponents lands seems blueish. Actually, this would probably need to be a bit more costly.

It's like the opposite of Benthic Explorers :P

Yeah, I could see the flavour of this working. It's interesting that it can /either/ play mana acceleration, /or/ mana denial, but not really both unless you want to play flash creatures in your opponent's upkeep.

As written, this can target lands that are already tapped. Intentional?

Oh yeuch. Well, now we know why this isn't a mechanic already.

I don't want to allow it, because then it's your-colour mana acceleration on your own lands.
And I don't want to disallow it, because opponents will just always tap in response.

Blehhh. Fixing this through changing the target is horrible. Least horrible is to make it cost mana to use, and then it isn't a mana-source any more.

Oh fiddlesticks. I think maybe of the competing horriblenesses, simplest is to make it target opponent's land.

I could see it as "T, Tap an untapped land an opponent control: Add ..." That way it's a mana ability, so your opponent can't respond. The only way is to not have an untapped lands, which is good for you anyway

Can you tap someone else's land as a cost? I thought costs had to be a thing you can pay already, not give you the ability to pay that.

Although 'sacrifice' is a bit of an outlier there, you can't just arbitrarily sacrifice stuff without an outlet... So maybe this could implicitly grant that ability?

@dude1818: Untap, upkeep, active player may now activate Undertow Mage. If they do not, other players may then activate their lands in order to prevent future activation of Undertow Mage. Move to Pre-combat main step. Active player may activate Undertow Mage...

It may work, but it's a book-keeping nightmare. Certainly not something Arena wants to deal with.

Maybe this should become something completely different?

You control target land until ~ dies?