Breathing underwater

A few years ago, an isolated tribe was found in the middle of South America. They seemed to be a normal tribe, but near their village is a lake with a special property- you can breath underwater inside of the lake. Naturally, the tribe views this lake as a sacred place, and refuses to let outsiders collect samples or analyze the "water". As nobody outside the tribe knew about this, the tribe was left alone to do their own business. Unfortunately, during the last 5 years, a strain of bacteria known as H. Wav affected this native population particularly hard, causing the whole tribe to die out. 2 weeks ago, a dehydrated hiker saw the lake and fell in while trying to get a drink. Instead of drowning, the hiker noticed that he was able to breathe. At first, the authorities didn't believe him, but they sent out someone just to check. Samples of the lake "water" are just now arriving in most major research institutions.

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What, if any, liquids can have an effect like this? The liquid must:

• Be easily mistaken for water (at least from a distance). It should be clear (or very close), and not have a strong odor from a distance. A slight smell nearby is fine, and it can taste however.
• Be non-toxic: an hour of breathing it every month for life should have no short or long-term effects. You should also be able to open your eyes, and any swallowing that occurs shouldn't kill you.
• It does not have to be naturally occurring. I'm not concerned with how it got there or how there's so much of it, as long as such a liquid exists.
• Bonus points if the liquid can support fish, plants, and other small marine life.