"Blood falls" in Antarctica. This water contains ferrous iron, which, combining with atmospheric air, oxidizes and forms rust. It gives the waterfall that blood-red colour
Source:
/r/interestingasfuck
/u/XyRow666
I honestly found this one googling around, but XyRow666 presented a far nicer collage than anywhere else I could find.
Roughly two million years ago, the Taylor Glacier sealed beneath it a small body of water which contained an ancient community of microbes. Trapped below a thick layer of ice, they have remained there ever since, isolated inside a natural time capsule. Evolving independently of the rest of the living world, these microbes exist in a place with no light or free oxygen and little heat, and are essentially the definition of “primordial ooze.” The trapped lake has very high salinity and is rich in iron, which gives the waterfall its red color. A fissure in the glacier allows the subglacial lake to flow out, forming the falls without contaminating the ecosystem within.
That's just what They want you to believe. The government is just trying to cover up the truth about the eldritch gods waking up now that their frozen prison is melting. /s
Edit: I put the /s there just in case. You never know what people will take seriously.
I was gonna make a joke about the real cause being prolific polar bear activity then I realized they're not in Antarctica and I would be corrected... On the internet. A fate worse than death.