Osorezan (meaning Fear Mountain), also known as the gate to Hell, is one of the most sacred places in Japan. Located in the Shimokita Peninsula, Tohoku, it’s a highly volcanic terrain, featuring a poisonous lake with high sulfur content, a river which is the Styx of Buddhism, yellow-colored fumes, boiling mud and strangely colored pools of nope.
It’s a rather barren area, the ground is all gravel and stone, but surrounded by small mountains covered with trees. It hosts a large temple complex and several statues. A few paths lead to the lake, swirling around memorials and offerings (lots of coins and toys) for the souls of dead children. There is no warning signs, or ropes, or anything that can indicate danger, so you have to watch where you walk if you leave the temple grounds, especially if you go behind the largest statue of Jizo. The ground is thin, and you can hear liquid boiling under your feet.
I recommend visiting it on a cloudy day, because its atmosphere is not that suited for sunlight.
Going there can be rather inconvenient, and from November to April there are no buses at all as the temple is closed, so go outside this timeframe. From Tokyo, take the shinkansen to Hachinohe, then change trains to go to Noheji, and finally to Shimokita, then take a bus from the Mutsu Bus Terminal (you can walk there). It will take you about 5 hours, and then add the 40 minutes bus ride to that. I recommend sleeping in Hachinohe or Noheji. There is only one bus that goes to Osorezan so it’s not hard to find, but I do recommend learning the kanji for it so you can recognize the bus: 恐山