Sanzu River

In Japanese Buddhism it is believed that to enter the underworld the soul first needs to find and cross the mythical Sanzu River (river of three crossings), which is the boundary between the worlds of the living and the dead. This river portal is believed to be located on a desolate yet holy volcano in northern Japan. Some believe the dead are accompanied on their journey to find the Sanzu River by three oni (underworld demons) and face many horrifying trials along the way including seas of blood and attacks by ghastly birds. 

From Tosa Mitsunobu's Juo-zu

There are three ways to cross the river, a bridge for the good souls, a shallow section for those who are neither good nor bad, and a deep section filled with poisonous snakes for the evil souls. After crossing the river, the dead enter Meido where there are a number of trials and they are judged as either worthy of going to Tengoku (heaven) or doomed to go to Jigoku (hell). If you are interested in more detail, yokai expert Matthew Meyer has written about this complex process here.

Scenes of Hell and Heaven' - Utagawa Kuniyoshi, 1835
The deity Jizo is believed to protect the souls of children who die before their parents. This is because it is believed they are unable to cross the Sanzu River and are destined to pile stones on its banks for eternity. You can read more about Jizo here.

'A Buddha in Hell' from the series 'One Hundred Pictures by Kyosai' - Kawanabe Kyosai
 'Holiday in Hell' - Tsukioka Yoshitoshi, 1868