The hook
A cliff of basalt columns so geometrically perfect it looks stacked by hand — 60-foot hexagonal posts a glacier sheared off and polished flat on top, so you can walk out onto the columns' honeycombed ends like paving stones.
01Why it's worth it
Devils Postpile is one of the world's finest examples of columnar basalt: about 100,000 years ago a lava flow pooled here and cooled so evenly it cracked into tall, mostly-hexagonal columns, "closely and perfectly fitted together like a vast mosaic." Then an ice-age glacier ground across the top and left the column-ends glass-smooth with visible striations — so the signature move is climbing the short trail to the top and standing on a tiled floor of hexagons the ice made. Two miles downstream, Rainbow Falls drops 101 feet off a lava ledge into the San Joaquin, and at midday the spray throws a full rainbow that names it. It's a small national monument (798 acres) but it packs the Eastern Sierra's best geology-in-an-afternoon, tucked in the Reds Meadow valley below Mammoth.
02How to experience it
Summer only — snow closes it most of the year, and access is a mandatory shuttle from Mammoth Mountain Main Lodge (roughly June–September; drive up over Tioga from the Bay). Do the Postpile loop with the climb to the polished top, then continue to Rainbow Falls (time it near noon for the rainbow). Pairs with a Mammoth/Mono Lake Eastern Sierra weekend.