At least 12 Sherpa guides died Friday on Nepal's side of Mount Everest when an avalanche buried them on the world's tallest mountain.

The death toll may go higher: The Himalayan Times reports that while 12 bodies have been recovered, an additional body "has been sighted buried in the snow," and that as the day ended another five Sherpas were still missing. CNN quotes a Nepalese Tourism Ministry official as saying at least four Sherpas were still unaccounted for. We will watch for updates.

Regardless of the final toll, it's the single deadliest day ever on Everest — surpassing the eight deaths in May 1996 when a storm struck. That tragedy was the basis for the best-selling book Into Thin Air.

According to Reuters, the avalanche "hit the most popular route to the mountain's peak ... between base camp and camp 1." CNN says the site of the disaster is about 20,000 feet above sea level. Everest's peak is an estimated 29,035 feet above sea level.

This is the climbing season on Everest, which more than 4,000 people have successfully climbed. About 250 have died on the mountain that borders Nepal and Tibet, Reuters notes. The Sherpas who were killed Friday and some climbers had in recent days been setting ropes, preparing camps and acclimating to the altitude, CNN reports.

While dangerous, Everest is not the world's "deadliest" mountain, according to various analyses. As The Daily Beast has noted, Nepal's Annapurna has a "death rate" of nearly 38 percent — or, as The Telegraph has put it, Annapurna has "the highest fatality-to-summit ratio of any mountain over 8,000 meters [26,247 feet]." While about 160 people have reached the top of Annapurna and returned, at least 60 have died trying.

Everest's death rate stands at about 6 percent. Other mountains with higher death rates than Everest, according to The Daily Beast's calculations, include:

-- K2, which straddles China and Pakistan (23 percent)

-- Nanga Parbat, in Pakistan (22 percent).

-- Kangchenjunga, on the border of India and Nepal (19 percent).

Update at 12:55 p.m. ET. Sherpas' Deaths Renew Ethical Issues:

All Things Considered just spoke with Grayson Schaffer, a senior editor and writer for Outside magazine who last year wrote a piece headlined "The Disposable Man: A Western History of Sherpas on Everest."

He reported then that:

"According to the , which keeps track of such things, 174 climbing Sherpas have died while working in the mountains in Nepal — 15 in the past decade on Everest alone. ... During that time, at least as many Sherpas were disabled by rockfall, frostbite, and altitude-related illnesses like stroke and edema. A Sherpa working above Base Camp on Everest is nearly 10 times more likely to die than a commercial fisherman — the profession the Centers for Disease Control and Prevention rates as the most dangerous nonmilitary job in the U.S. — and more than three and a half times as likely to perish than an infantryman during the first four years of the Iraq war. As a dice roll for someone paying to reach the summit, the dangers of climbing can perhaps be rationalized. But as a workplace safety statistic, 1.2 percent mortality is outrageous. There's no other service industry in the world that so frequently kills and maims its workers for the benefit of paying clients."

Note: As we pointed out, he wrote that before today's tragedy on Everest.

Today, Schaffer told NPR's Audie Cornish that "there's a sense among climbers that it's acceptable" for Sherpas to take such risks "because it's part of their culture." But, as he said, "there are some real ethical concerns" about whether that's a moral position.

Much more from his conversation with Audie is due on Friday's All Things Considered. Click here to find an NPR station that broadcasts or streams the show.

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