I STEPPED into Walden Pond and stood for a moment watching a light rain that had begun to make rings on the surface. Then I adjusted my swim goggles and pushed forward into the water. The water was a pale green, and for several strokes I watched the pebbles, then a log, as I glided overhead. Then the water turned a dark green, almost black, and I was into the long swim across.Once a lifeguard, I was doing what I advised others not to do: swim alone in deep water. On this afternoon in late August, there were only a few other swimmers, mostly at the beach end of the pond. One or two other distance swimmers had angled off to my right. For a few seconds I considered the risk. I am a good swimmer... it was probably less risky than driving my car, I told myself. I switched from breathing every third stroke to every other stroke. I felt comfortable, relaxed. I would be in the lake for a while, and it is a good time to think. When I'm alone at Walden, I always think about Thoreau. The summer before my senior year in high school in California, I picked up a copy of "Walden." This was in the 1960s. After that, I started looking at things differently. My parents and most of my teachers didn't have much good to say about Thoreau. Except my English teacher. She had us read "Civil Disobedience" and passages from "Walden." "The mass of men lead lives of quiet desperation," Thoreau said. I could see that. I was also beginning to feel the anguish of Vietnam. When I came to college in Massachusetts, one of the first things I did was hitchhike out to Concord. I followed Thoreau's path to the pond and left the obligatory stone at the site of his cabin. In winter, with my roommate, I returned. We ran down the hill and slid out far on the ice. Below me the black water, at first chilly, was pleasantly cool. I switched, for a rest, to the breast stroke, then took my bearings: about a third of the way across. Then I returned to the crawl and peered down deep into the cool darkness. How deep is it here, I wondered? I thought of Thoreau in winter, lying on his stomach on the ice, peering down through the holes he had chiseled. In 1846, using a stone tied to a cod line, he made over 100 soundings, carefully surveying the shape of the bottom. Walden Pond wasn't bottomless, as some in the village had rumored. But it was deep, exactly 102 feet at the deepest point where the lines of the greatest length and width crossed, he was surprised to find. SPRING fed and deep, Walden Pond still replenishes its waters, keeping them clear and cool despite the thousands who may swim there on a hot summer day. It is also deep and cool for trout. I've seen good-size rainbow and brown trout, small-mouth bass, yellow perch, and sunfish taken from the pond. The trout are a recent addition. They do well, apparently because of the pond's cool depths. Thoreau found no trout, but in "Walden" he wrote of pickerel, yellow perch, "pouts," breams, eels, and shiners. He was fascinated by the gold and emerald colors, as well as the design of the pickerel, of which he noted three different varieties. I think of him fishing in summer, the surface mirror-like, his line falling into the sky. Or in the winter when he would come down from his cabin with an ax and fill pails with water. Opening a window into the light-infused shallows, he would gaze "into the parlor of fishes." He wrote: "Heaven is under our feet as well as over our heads." Thoreau had a keen eye for the creatures of his watery kingdom, the large and small animals seen only when it is quiet: otters, muskrat, mink, mud turtles, and fish hawks. Each October, a solitary loon arrived. Following this marvelous creature in his rowboat, he played hide-and-seek with it, as the loon tirelessly dived and reappeared. Once with my family in our canoe, we watched a small water snake cross the lake, only the tip of its head showing, making a delicate V-shaped wake behind it. SWIMMING alone far from shore is a time for deep thoughts. When I had swum further than halfway, I thought: At 45, I'm past halfway in another way, too. It's taking me so much time, it seems, to find out who I am, what I'm really good at, and where I'm going. I'm still struggling to pay my bills. Bad things have happened, but I'm still around. I have a family, and health. And I'm learning to cherish the connections I have with those around me. Also to relish the times to be alone, to take my own sounding s. My body became warm, almost too warm. Gray sky and flowing, marbled water rocked back and forth in my vision as I turned my face to the side for air. Ahead my pale hands entered the cool darkness and pulled streams of tiny bubbles under me and back. The cadences of my stroke were the cadences of prayer. Then I was just swimming and not even thinking any more, until suddenly the light green of the bottom displaced the darkness. I was back again. Terra firma rose to meet me. A sunfish darted from my fingers. My hands brushed the silt and sandy bottom. I sat half-submerged in the shallows. The commuter train roared by on the far embankment. Then it was quiet again, except for my labored breathing, which gradually subsided. I could hear the tiny drops of the rain. I looked at the sky, which was starting to brighten, then at the rich green of the white pines on the eastern shore. The beach was empty. A mother with two young children walked up the hill. To my left a solitary person was fishing. My eyes searched the opposite shore, my starting point, for t he blue towel I tucked under an overhanging rock. I could just see it. After resting, I returned.