Lit.: River of Three Crossings
The Sanzu river is the Japanese Buddhist equivalent of the River Styx, which the Japanese believe the dead must cross on the seventh day after their death. The crossing can be easy or difficult depending on the deeds performed by the deceased during their lifetime.
The riverbed of the Sanzu no Kawa is reserved for unfilial children who die before their parents, thereby causing their parents great sorrow. This place is called the Sai no Kawara (賽の河原), or "Children's Limbo." The child is condemned to remain there to build a tower of stones, pebble by pebble, for the memorial service of his or her parents, but the pagoda is knocked down by a demon before it can completed, and the child must begin again. The cycle continues until the child is finally saved by the Kṣitigarbha bodhisattva.
There are many Sai no Kawara sites located throughout Japan where grieving parents go to pray for the soul of their child and to console their own sorrow, that their child may rest. One of these which goes back to at least 1658 is located on the bank of Lake Ashi in Motohakone, at the foot of the shrine gate of Hakone Shrine.