Lassen Peak is the southernmost of the big Cascade volcanoes, and it is the one that most recently reminded California it was alive. Between 1914 and 1917 it erupted again and again, and in May 1915 a blast and a mudflow sent ash far to the east and stripped a long swathe of forest from its northeastern side. The scar is still visible as the Devastated Area, now slowly greening.
The park highway is one of the great short rides of the West. It climbs from forest into rock and snowfield, loops around the shoulder of the peak at well over 2,500 metres and comes down the other side past lakes so clear they look shallow. The corners are steady and well engineered, more flowing than technical, but the surface can be broken where frost has worked at it, and in early summer the walls of snow on either side can be taller than your head. The road is closed by snow for much of the year and usually opens only in late spring or early summer, so check before you plan around it.
Stop at the main hydrothermal area, Bumpass Hell, if you have the time and decent boots. A trail of a few kilometres leads to a basin of boiling pools, steaming vents and mud pots that smell of rotten eggs and hiss like a kettle. It is named after an early guide who broke through the crust and badly burned his leg, which is all the warning you need to stay on the boardwalks. Closer to the road, Sulphur Works lets you see and smell the same thing from a few steps away.
Lassen sees far fewer visitors than Yosemite, and that is part of its charm. Mornings are calm and clear, and afternoon storms can build quickly over the summit in July. There is little in the way of services inside the park, so fill up in Chester, a small lakeside town with fuel, food and a couple of places to sleep, or at the far side of the park before you climb. Bring layers, because it can be warm at the lake and near freezing at the top on the same day.