The Shingon sect is very different from the Zen that many of us associate with Japan, not to mention the Jodo Buddhism of recitations. It is most similar to the swirling, electric intensity of Tibetan Buddhism (not surprisingly, since that is the form of Buddhism which was dominant in China when Kukai visited). Those in this esoteric sect believe that the whole world is a mandala, or drawing of enlightenment. They practice Tibetan mudras, or hand gestures; they recite mantras to draw closer to the secret rhythm of the universe; and they believe that if you master many secret, ascetic practices, you can attain enlightenment in a single lifetime. "Buddha can be here in this very body," as one Koyasan monk explained to me.

In 835, the first temple that Kukai built, Kongobuji (or Diamond Peak Temple), received official recognition from the emperor, and in the following month, Kukai died, leaving his followers to continue settling the mountain. Eighty-six years after his death, another emperor gave him the honorific title of Kobo Daishi, or "Great Master Who Spread the Buddhist Teaching." To this day, Kongobuji oversees the 3,600 Shingon temples and more than 10 million practitioners scattered across Japan.

Perhaps the first thing you notice when you get to Koyasan is a dark, collected power that could not be further from everyday Japan, in which even most Buddhist temples are unexceptional places associated with funeral rites. Manners on the mountain are brusque, unfussy; the faces I saw as soon as I arrived were wild-bearded, grizzled—much rougher than in manicured, eyebrow-plucked Kyoto. At the central tourist information center, where I booked my temple rooms, the staff were not women, who lavish helpful attention on foreigners in every other such office in the country, but men, many of whom spoke no English and did not look at all delighted to see visitors.

In cities like Kyoto, a recorded voice sings out "Welcome" when you enter even a video store; in Koyasan, the message, refreshingly, seemed to be, "Keep your distance. And remember where you are."

Yet very soon that air of weightedness and remoteness began to feel like the source of the area's power. Even now there are, by one count, more than two thousand stupas, shrines, and pagodas on top of the mountain, and on the cable car up, the final announcement had told us, in Japanese and English, that there are fifty thousand cultural and historical treasures among the tall cedars and cypress trees. A sign at the entrance to the cable car had proclaimed that Koyasan, as of 2004, had been named a UNESCO World Heritage Site. And as I began walking around the settlement my first morning, I heard a solemn gong from within a temple nearby, and looked up to see a handsome young Japanese man dressed from head to toe in pilgrim's white, with a ceremonial topknot to keep his long hair under control. As I wandered along the main street—from the graveyard on one side of town, heavy with tombs and weathered headstones, to the bright temple compound on the other—I noticed that many of the visitors were groups of pilgrims, often elderly, also wearing white shirts, on the back of which was written, WE DEVOTE OUR LIVES TO KOBO DAISHI AND WE ALWAYS WALK WITH KOBO DAISHI. Wherever they visited, they stood and let out loud chants while a leader hit a bowl steadily, making a clacking sound, or shook what sounded like bells.