Aletschgletscher, at 27km is the longest glacier in the Alps. It is located in the canton of Valais in Switzerland. The Aletsch flows south and then west from the Jungfrau region, which has numerous well-known peaks, the Eiger (3970m), Jungfrau (4158m), Aletschhorn (4193m) and Monch (4107m). It is fed by three smaller glaciers, the Grosser Aletschfirn, Jungfraufirn and Eroigschneefald which meet at Konkordiaplatz (there's a SAC hut called the Konkordiahutte close by). Like most glaciers, it has been receding over the past few centuries as the average temperature rises globally. The glacier feeds the Massa river which is a tributary of the Rhone.
The glacier lies south of the Jungfrau/Interlaken region and north of the Goms valley. The ice itself is quite dirty, there is a layer of dust on top of it and it does not look white except after fresh snowfall. Looking at it brings home the amount of pollution even in a lightly industrialized area like Valais.
The entire Aletsch region (along with the Bietschhorn and Jungfrau region) was designated a UNESCO World Nature Heritage site in 2001 and the local tourist industry has made the most of it. You'll find the sentence "27km eis, eine stuck" practicaly everywhere. Like most popular destinations in Switzerland, the area is saturated with kitschy souvenir shops. However, the mountains and the views are stunning and mostly pristine.
There is a fine medium difficulty hike that is an ideal tour of the glacier. It starts out in Fiesch (Valais, 1049m) which is served by the Furka-Oberalp line. From Fiesch, a cable car runs up to Kuhboden (2212m), there is a steep, well maintained trail as well. Kuhboden is where the really nifty part of the hike starts. The best route is to climb up to the Eggishorn (2927m), there is a cable car that will take you almost to the top, but the stark rocky landscape and the increasingly better views of the Goms make the trek up quite worthwhile. From the Eggishorn on a clear day there are fine views of the Matterhorn and the Mt. Rosa group (45km away). The panoramic view includes the mountains in the Jungfrau region with their stunning hanging glaciers, the peaks along the southern flank of the Goms, and the Aletschgletscher itself. The Eggishorn peak is rather broad and you can spend an entire morning there taking photographs from various angles.
From the Eggishorn, hike down to Marjela, a tiny beautiful valley with a small Alpine tairn. From here the hike runs parallel to the glacier at 2200-2300m and affords good views of the Junfrau peaks Gross Fusshorn (3627m), Geisshorn (3740m) and Aletschhorn. One of the highest pine forests in Europe lies along the trek, you can see beautiful stunted pine trees along the route. The trail finally crosses over to the north slopes of the Goms and enters the town of Bettmeralp (1957m), where there are frequent cable cars down to the Betten train station. On the whole, this is a very good one day hike with some stunning views on a good day. The trail is very well maintained, footing is quite good, and there are few spots to trigger vertigo. I completed this 20km hike in 2002 on a sunny day.
Sources: Swiss topographical commission's map 264T