Take, for example, this account by the adventurer, F.A. Mitchell-Hedges, who spent the years 1919 to 1921 in Central America, especially Honduras.
I collected evidence relating to unknown tribes. I heard the story of the River of Blood. Indians living in the district say that a crimson stream gushes from the bowels of the earth and anybody unfortunate enough to see it will die at full moon. They believe that all those people who have lived corruptly on earth turn into bats and when they die are condemned to a perpetual hell under the rule of a demon in the shape of a colossal bat. They live in the underground regions from which the River of Blood flows. These unclean spirits wage perpetual warfare and the foul blood that runs from the mangled bodies bursts from the cave in a gory stream.Reference: F. A. Mitchell-Hedges (1954), Danger My Ally, Elek Books, chapter 2(i)
The true explanation is simple. The subterranean stream runs through red earth similar to the Devonian strata and emerges crimson.
(p 106 of the 1956 Pan edition)
PS: I have since learned that the Popol Vuh, a book containing the classic mythology of the K'iche Mayans of Guatemala, refers to the bat-god, Camazotz, who presents as a giant, human-headed bat, who dwells in the Bat-house of the Underworld, conceived as a labyrinth of caves filled with giant bats. The explanation of the River of Blood no doubt relates back to this myth.