And praying while watching the sunrise from the summit
And here are our answers.
Mt. Nantai - Image by Asa-moya
Why is the mountain sacred?
It is not surprising to learn that ancient human being, regardless of their ethnicity, cultural background, and geographic location, were all in awe of mountains and considered them sacred. This sort of idea called “animism” was initially developed by a German scholar - Georg Ernst Stahl, and then redefined in English by Edward Burnett Tylor. Personally, I don’t feel like describing this as a religion, rather, I would call it faith. And to me there is a significant difference between the two.
A mountain can be worshipped and considered sacred because of its shape, frequency and magnitude of volcanic eruptions and mythical significance. Mt. Olympus and Mt. Ida are the highest mountains in Greece and on the island of Crete, respectively, and both of them are sacred mountains of Greek gods.
The same principle applies to ancient Shinto called Koshinto (古神道) in Japan. Inhabitants of Japan during the Jomon period were in awe of mountains where they dwelt, hunted animals, went fishing, and harvested nuts. Without a food supply from the mountain, they had no life, or their life would have been much more difficult.
After obtaining a variety of things that their mountains had to offer, how did they feel? It’s only natural that they spontaneously expressed their gratitude to the mountains by organizing ritual ceremonies, however primitive and simple they might have been.
Then it is not surprising either to further imagine that they decided to have rules within their community not to over-hunt and poach animals living on the mountain. They could have decided to distinguish the area of their everyday life from the hunting grounds on the mountain, which they might have started to call a “sacred ground” or “sanctuary”.
Image by Marx Pix
What they did next was to mark a ceremonial entrance to the mountain. Later on, that hut turned into a shrine and the gate - torii (鳥居) was painted in vivid red to be easily noticed. That’s why shrines tend to be built at the border between everyday dwelling area and sacred places.
Now you know why the entrance gate to Mt. Nantai is at Futaara shrine's Chugushi shrine (二荒山神社中宮祠). The objective of Futaara Chugushi shrine is to restrict the entry to the holy mountain - Mt. Nantai, where Okuni (or Ookuni no nushi or Oonamuchino-mikoto: they are all the same god but with a different pronunciation of his name) is worshipped.
Entrance to Mt. Nantai
What does one pray for or pray to during the climb?
As Ms. T. already explained in her comment, it could be anything. It’s all up to you - Okuni is a very open-minded god, you know. And because you pay (= donate) 1000 yen (= approx. 10 bucks) for climbing Mt. Nantai at night, you have the right to make your own wishes, no matter how evil or dirty they are.
Now I hear you say, “this is not what I wanted to hear.”
OK, OK, there is more.
There is a common expression while climbing a mountain, especially when climbing Mt. Fuji. It’s called “rokkon shojo" (六根清浄). It’s theoretically a Buddhist term. “Rokkon” means “six senses [visual perception, hearing, olfaction, taste, sense coming from skin (touching, temperature, pain), and consciousness]. “Shojo” means purification. When put together “rokkon shojo” means purifying klesa (poison) inside your heart that was produced through the six senses (rokkon).
To purify your spirit and remain as pure as possible, you got to avoid seeing, hearing, sniffing, tasting, touching, and sensing evil things around you. By repeating the term “rokkon shojo” while struggling to climb a holy mountain, your spirit will be purified. The harder the trek, the purer your spirit becomes. Basically you are virtual-experiencing the discipline that yamabushi (山伏) have been practicing for centuries. And then, the full-of-pain endeavor of purifying your spirit culminates in exposing yourself to the holy shower of rising sunlight on top of the sacred Mt. Nantai!
In order to get rid of your klesa (i.e. dirty mind and evil spirit, you name it) you should climb at night, and that’s why we start climbing at midnight. We should reach the top before the sun rises. Does it make sense now? I hope it does.