Las Vegas police spotted the reflective shape during a search and rescue mission over the weekend in a hiking area an hour north of Sin City.
"We see a lot of weird things... but check this out!" the force posted Monday on social media.
The rectangular structure is the latest to baffle the internet after they began appearing in places around the globe in 2020.
The first monolith, which was discovered in the Utah desert as the world was grappling with the Covid-19 pandemic, quickly caught international media attention.
Many noted the object's similarity to alien monoliths that trigger huge leaps in human progress in Stanley Kubrick's classic sci-fi film "2001: A Space Odyssey."
There have since been sightings in the Netherlands, in Poland, Romania, Canada and Britain, sparking the inevitable websites dedicated to tracking them.
The Nevada find, which the police posted on their social media feed on X, generated a bevy of comments ranging from awestruck speculation to the downright dismissive.
"Clearly Aliens!" wrote user @NWK1WAYS.
"Let's crack it open may be prizes inside," suggested @Nstynick.
"Are we really gonna do this whole thing again?" asked a weary @cdharris19. "Just remove the trash, find who dumped it, and fine them."
