Vernal Falls (flickr/joe shlabotnik)

Witnesses in California's Yosemite National Park watched as three hikers were swept over the 317-foot Vernal Falls on Tuesday.  Though the three are feared dead, authorities are calling their probe a search and rescue operation.

Eyewitnesses said that a man and woman climbed over the railing and appeared to be taking pictures when they slipped on wet rocks off of the Mist Trail and tumbled into the fast moving waters of the Merced River above the falls.  A third man then jumped into the waters attempting to rescue the couple.

Witnesses looked on in horror as all three were then swept over the falls into a rock-strewn pool over 300 feet below.

30 search and rescue personnel were combing the region on Tuesday afternoon following multiple emergency calls from eyewitnesses.  The search was called off for the night, but resumed on Wednesday.

Park spokeswoman Kari Cobb said the footing in the area can be treacherous because of the persistent mist in the air.

The portion of the trail up near Vernal Falls is called the Mist Trial, she said. It's a very wet area. The water from the waterfall actually flows onto the trail so those rocks are wet and they can be slippery.

After a record winter snowfall, Yosemite received 199 percent of normal snowpack which has led to higher water levels than normal.  While the views of the falls are spectacular, according to Cobb, river currents are treacherous and at least 8 people have died in the park this year.

The path to the top of Vernal Falls, a three-mile trek with a 1,000-foot rise in elevation, is one of the most popular hikes in the park. At the top, a guardrail separates visitors from the dangerous river currents, and signs in multiple languages warn visitors of the danger created by slippery boulders. Published accounts say that 13 people have died by falling over since records began being kept in the 1920s.

Rangers closed Mist Trail following the incident.