Every year I climb Mt. Diablo (12 miles, 3,500' elev. gain) on or around my birthday, to calibrate my physical condition and chart its inevitable decline. It's Christmas and I still haven't done this "reference hike". It occurs to me that I could spice things up by doing the hike at night. The thrill of being in a wild, remote place at night has been documented here and here.

Now, there are mountain lions on Mt. Diablo, and they're nocturnal predators. This concerns me a bit. But I can visualize fighting off an attack, and isn't this the first step to actually doing it?

I choose Christmas day for my attempt: stout exercise is a good antidote to holiday bloat and lethargy. The Clayton forecast is for partly cloudy, lows around freezing. There will be no moon, which is a slight negative - moonlight wouldn't suffice for walking, but it would reveal the contours of the mountain.

I pack up various gear, set out for Clayton at 5:30 PM, and am on the trail at 6:30. I start off with fleece and down, but quickly get hot and strip down to my polypropylene long-sleeve undershirt.

It's pitch black.

Selfie. Too dark for any other pictures.

When I turn off my headlamp, the mountain is a pure black shape against the faint glow of the sky. My LED headlamp works well, but I have to decide where to direct its bright spot. Where the trail is rocky, I need to aim it 3' in front of me, to avoid stepping on loose rock. In other places I can point it 20' ahead and get a bigger picture.

I feel good. My balance is a bit wobbly because of the disorienting effect of the headlamp. At first I occasionally scan for the glowing eyes of a mountain lion, but forget about this quickly. I hike methodically: safety first, go quickly when possible, conserve energy.

There has been a lot of rain recently. Back Creek is babbling loudly, and several smaller creeks cross the path. The trail is damp, and slightly muddy here and there. Plants impinge on the trail more than usual.

I reach Murchio Gap in 1:10, a bit slower than typical daytime pace. I've brought a half liter of water, cashews, and chocolate chip cookies.

The second leg - Bald Ridge Trail - is the high point of the evening. The manzanita trees, lit by the headlamp, look ghostly and macabre. Various things reflect the light and seem to glow in the dark: lichen patches, white quartz, manzanita leaves on the ground. The image is mostly black-and-white (because of low light) but with occasional hints of green. A reflection off my glasses frame creates the illusion that I'm walking toward a brightness, like a near-death experience. It's all kind of psychedelic, and reminiscent of "Blair Witch Project".

I worry a bit about headlamp longevity. I've brought 2 headlamps, but I'm not sure how long the batteries last. Loss of light would mean stopping and waiting out the night. But this doesn't happen.

As I approach Prospector Gap, a mist appears in the air. I look up and see that the summit is enveloped in dark clouds. After a few minutes the mist thickens to a heavy, vision-impairing fog. I can't see beyond about 10 feet.

By the time I reach the gap, visibility is down to about 5 feet. I can't see anything except the trail directly in front of me. At this point the hike becomes claustrophobic and unpleasant, and perhaps it would have been prudent to turn around. Losing the trail, or taking a wrong turn, would require stopping and waiting for dawn.

Of course, I decide to continue. The factors in this decision:

• The trail itself is pretty well-defined; the odds of losing the trail completely are small.
• There are few side-trails, and I know where they are; the odds of taking a wrong turn are small.
• I'm extremely familiar with this trail. I know its general contours, and I know scores of small landmarks: rocks, switchbacks, etc.; the odds of becoming disoriented are small.

So I keep going, though the thrill of nighttime hiking is replaced by tension and paranoia. There's cell phone reception the whole way, and I maintain text-message conversations with friends.

The trail from Prospector Gap to the summit is meandering and largely horizontal. By looking at satellite photos I've identified a possible short-cut that involves scrambling up a steep rocky slope, about 40' wide with trees on either side. I reach this point and give it a try, but it's immediately clear that it's infeasible because of the fog, and I go down and rejoin the trail.

The fog gets thicker and thicker. I can barely see anything. I slog on, and eventually reach the summit at 9:30 PM. Large featureless areas like the road and parking lot are frightening because of the fog. One false move and I'm lost and disoriented. I stay to the edges and try to recall the features to look for.

At the summit, the wind picks up a bit, and it's lightly snowing. There's snow along the sides of the road.

The beacon at the summit is on, though you can't see it beyond ~50'. I sit on the stone bench at its base and rest for 10 minutes. The drinking fountains at the top aren't working, and I only have a bit of water left. Fortunately I haven't sweated much.

As the beacon rotates, the fog alternates between gray and a hellish red. I let the isolation, strangeness, and potential danger of my situation wash over me. Oddly, I enjoy things like this.

I start down. It's very hard to navigate. I miss the Summit Trail turnoff, then backtrack and find it. This pattern repeats several times on the way down; it takes several tries to locate the Bald Ridge trail at the gap.

Below the gap, the fog gradually clears and I breathe a sigh of relief. On the Bald Ridge trail, I can see a vast band of city lights in the distance, from S.F. up to Benicia. I briefly turn off my headlamp and enjoy the view: the totally black bulk of the mountain against the background of amber lights.

The descent is physically hard. On the way up I've accumulated lots of potential energy using muscle power; on the way down all this energy is converted to impact on my knees and back. Some tweaks of discomfort from both, and my feet get progressively sorer.

I'm organized and business-like. A few slips, and a scary ankle-twist with about 100' to go. I'm back at the car at 12:30 (3 hours up, 2.75 hours down), and am back home at 1:15 AM.

Summary: I'm very glad I did this but don't plan to repeat it. Night hiking is, I think, best done without a headlamp. This requires a fairly smooth trail and some source of light, either moon or reflected city lights. Also, fog is a game-changer.

Health report: soreness verging on pain in both feet (arthritis). Still sore the next day. I hadn't hiked in a while so legs got a bit tired and sore (OK the next day).