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Two tourists view and photograph the Mendenhall Glacier from inside the visitors' center

The Mendenhall Glacier is a tongue of ice, stretching 12 miles and reaching 1-1/2 miles across the Mendenhall Valley, with ice 400-800 feet deep. Twelve miles is about the same distance as cruise ship passengers travel from the dock up to the glacier.

Although it's only minutes from downtown Juneau, a trip to the glacier transports visitors to a primordial past, when much of North America was buried under ice.

Naturalist John Muir named it the "Auk Glacier" after a local Tlingit Indian village when he visited Southeast Alaska in 1879.

In 1892 the name was changed to honor Thomas C. Mendenhall, superintendent of the U.S. Coast and Geodetic Survey. His administration surveyed the international boundary between Alaska and Canada.

Since the mid-1700s, the glacier has been retreating. Before 1750, the face of the glacier reached two miles farther down the Mendenhall Valley. Theories differ, but some suggest that a slight change in climate could make it advance again.

It takes the ice about 250 years to flow from the Juneau icefield (which is itself a glacier) to the terminus at Mendenhall Lake.

The vegetation has filled ins the landscape surrounding the Mendenhall Glacier Visitor Center since it was built 40 years ago.

The most intense blue colors occur in crevasses and when ice breaks off, or calves, from a glacier's face. The blue color fades as the ice is exposed to air and the crystalline structure begins to break down.

As the ice breaks down, it becomes porous, and allows all wavelengths of the color spectrum to pass through, creating a white appearance.

Visitors to the Mendenhall should not let gray skies discourage them. Glacier viewing is often best on overcast and rainy days.

Commercial operators who have special-use permits for the Tongass National Forest (in which the Mendenhall Glacier lies) offer a variety of tours in the summer, including hiking, biking, rafting, canoe and kayak trips and bus, van, taxi and shuttle tours. Experienced guides pilot large rubber rafts down the glacier-fed Mendenhall River or canoes and kayaks on Mendenhall Lake.

Thousands of visitors each year also take flightseeing tours by helicopter or fixed-wing plane. Some land on the Mendenhall and other Juneau Icefield glaciers.

Earl Hurting, an interpreter for the U.S. Forest Service at the Mendenhall Glacier Visitor Center helps Sharon Oldhamm, right, and Elena Yasno, both visiting from California, spot mountain goats from Photo Point

The 1/2 mile Trail of Time, a self guided interpretive/nature trail meant to be enjoyed at an unhurried pace, circles up and around the Visitor Center. There's also the fully accessible 1/4 mile Photo Point Trail, which overlooks the glacier and Nugget Falls.

There is a $3 entrance fee for the visitor center, which is open from 8am to 7:30pm daily May through September. The Alaska Natural History Association operates a bookstore in the visitor center