That time, Lavin says, he didn't realize the snake was on the trail until he had stepped over it. ``I think that made him mad, because he was still pretty tough and surly last week.'' That surly little fellow might be only detail Lavin has missed on his strolls up and around the mountain. Of Mount Diablo's 20,000 acres of hiking trails, there is no place that this 48-year-old attorney from Concord hasn't seen -- and seen again. When he isn't toiling in his day job at his Walnut Creek law practice, Lavin is a walking, talking history lesson on Mount Diablo, leading hikes for the interpretive association, the Mount Diablo Group of the Sierra Club, and this spring, with Concord Leisure Services. As part storyteller, part naturalist, Lavin discusses the mountain's history and its geology -- throwing in a little botany and zoology along the way.

``During your average hike on the mountain, we may only go a couple of miles, but we will travel through 175 million years of history,'' he says. ``People are amazed that our little mountain has such an impressive geologic life span.''

In his quiet, folksy manner, Lavin is able to deliver fascinating lectures on why the wildflowers grow, explain the nature of frogs to the ponds and point out the skeletons of manzanita trees burned in the huge 1977 wildfire that blackened 6,000 acres on the mountain.

Despite the time he spends on the trail, he says that he is always surprised by the unpredictable mountain, from the animals that live there to the forces of nature that toy with hikers. ``A couple of years ago, while returning from a hike to the waterfalls, we heard a boom in the distance,'' he says.

``I commented that it sounded like thunder and we'd better hasten back to the cars. A lady on the hike who had just moved here from the Midwest said nonsense, she knows what thunder sounded like, and what I had heard was just a sonic boom.

``Five minutes later, a bolt of lightning struck not 50 feet behind me on the hillside. I guess I showed her.'' Lavin's hikes have also fallen victim to El Nino.

``Back in 1998, on a hike to Frog Pond, Mountain House Creek decided to jump its bank after 2 million years -- just in time for my hike -- and took out the trail. I got everyone across the ravine where the trail had once been, but then had to go back and retrieve people's boots, (which) had been sucked off in the mud as they crossed.''

Lavin has had his share of encounters with tarantulas, 200-pound feral pigs and an occasional bobcat, coyote and fox. However, in 21 years of hiking, he's never met up with a mountain lion.

Some of his animal encounters -- while fun -- can be somewhat embarrassing, he says. ``During a hike for Concord Leisure Services, someone asked me to identify a bird. I looked up but kept walking and stepped on a snake,'' he says. ``So much for bird watching.''

Though Lavin sounds as if he's been a Mount Diablo hiker for years, he isn't. ``People measure their Mount Diablo knowledge by the years before the fire and after the fire,'' he says. ``The real old-timers know of the mountain pre-fire. While I know a lot about the mountain and its history, I didn't set foot on it until a few years after the wildfire.'' When Lavin moved to Northern California 21 years ago, he first settled in Albany. ``The major scenic attraction over there is, of course, the bay, with Mount Tamalpais in the background. When I moved to central Contra Costa County two years later, the landscape was dominated by just one major icon -- Mount Diablo. I felt compelled to learn more about it.''

He gathered most of his information from William Brewer's ``Up and Down California'' and through research at local libraries.

Even so, Lavin says it wasn't love at first sight. ``I thought it would be full of people clogging up the place,'' he says.

``I was wrong. Even these days, with 20,000 acres to explore and attracting one million visitors a year, Mount Diablo is still a wonderful place to get away from it all,'' he says.

``Legend has it that on a clear day, you can see for 40,000 square miles of land and water from the top of Mount Diablo -- the place of most visibility in the world,'' he says. ``While that is debatable, what is true is that on a clear day, you can easily see all the way over to Mount Lassen, and pretty far south.

``Mount Diablo really is our landmark,'' he says. ``You can see it from far away, and it identifies this region of the state. But it is just as beautiful up close.''

Angry rattlesnakes and all.