It started just over a month ago with a handful of anecdotal accounts of a strange phenomenon at one of America’s national treasures.
As the weeks unfolded the chorus of concerned visitors grew louder and louder.
When spooked tourists stopped showing up altogether, local guides and officials joined the refrain.
Something strange and frightening was happening at the Grand Canyon.
Its signature echo had fallen abruptly and inexplicably silent.
For weeks, tourists had reported their astonishment when – instead of the booming return they’d expected – their shouts and songs were met with absolute quiet from the canyon floor.
Philadelphia visitor Saul Hazey spoke for many when he said; “ït’s just so unexpected and, frankly, unnerving.”
“If you can’t count on an echo, what can you believe?”
Or as San Francisco schoolgirl Crystal Meff put it succinctly; “it just fully freaked me out.”
The majestic canyon – 277 miles (446 km) long and up to 18 miles (29 km) wide – is renowned for its powerful echo.
The average depth of about 1 mile (1.6 kilometres), its rocky walls and relative quiet provide a perfect crucible for sound to reverberate.
And that’s why scientists remain puzzled.
Professor Rod Handler from Northern Arizona University, just 74 miles (120 km) away in Flagstaff, has led a team taking audio samples at the canyon for the past week.
So far, he says, he’s none the wiser.
“I was sceptical there was anything to investigate, frankly,” admits Professor Handler.
“But whatever’s causing it, the phenomenon is very real.
“The area’s birds and animals are noticing it too and they’re behaving abnormally.
“We’ll keep investigating until we get an answer.”