Panum Crater is located south of Mono Lake in The Eastern Sierra Nevada Mountains. The crater is not so much a crater as it is a rhyolitic lava dome. It is also known as an obsidian plug. In very basic terms, it's a volcano formed by very viscous lava. What it really means from a hiking perspective is that the Panum Crater provides an unusual place to hike.
For this area of the Eastern Sierra, Panum Crater really isn't all that unusual. The difference between Panum Crater and some of the other domes in the area is that Panum Crater offers an easier way up. Some of the other craters in the area are very steep and covered with pumice. If you've ever hiked in pumice, you take one step up and slide two or three steps back. Next to mud, pumice is my least favorite hiking surface.
Panum Crater should be accessible all year around. It doesn't snow a lot here, so unless the winter has brought unusually heavy snowfall to the area, you should be able to hike the trail. From the trailhead coordinates the trail to the top of the dome is relatively gentle. Along the way up the trail there are piles of black obsidian. There are a few spots of obsidian elsewhere on top of the dome, but the bulk of the obsidian is early in the hike. Continuing past the top of the trail take you to some very good vantage points of Mono Lake, which lies just below the northern edge of the dome.
If you're not familiar with Mono Lake, it is a source of water for Los Angeles. Water is pumped from Mono Lake into the Los Angeles Aqueduct. The L.A. Aqueduct was completed in 1913. Since then it has a political hot potato. One of the first byproducts of the aqueduct was the diversion of the Owens River away from Owens Lake, 100 miles south Mono Lake. The result was that Owens Lake dried up. Later, the water level in Mono Lake started dropping. L.A. was drawing out of the lake faster than the area streams could refill the lake.
Over the past 20 years L.A. has not been allowed to draw as much water from Mono Lake as they had in the past. The result has been that the lake has been rising gradually. In fact, the water level has risen so much that Mono Lake's famed Tufa formations are all being reclaimed by the lake. If you are not familiar with Mono Lake's Tufa formations, look at the lake photo on Pink Floyd's Wish You Were Here album or what the movie High Plains Drifter. Both feature Mono Lake's Tufa.
When you're done exploring Panum Crater head back up to Highway 120. Drive west to Highway 395. Head north on Highway 395 past Lee Vining and take the turnout down to the lake. There are still exposed stands of Tufa there, although they are nearly as majestic as they were when the lake level was lower.
Check out the Official Route and Triplog.