Entering the colossal grove of giant sequoias in Calaveras Big Trees State Park to find the stump of the Discovery Tree is like creeping through a beautiful mansion, looking for a body. From the parking lot off Highway 4 — where yellow school buses dusted with pine needles unload excited fifth graders — one of the most stunning hikes in California leads into the forest. The gentle 1.5-mile North Grove loop passes by towering sequoias, some wider than a house, and older than the Roman empire. There are the Siamese Twins — two sequoias that grew so close together they form a single mass for the …