Mention the words Phnom Penh and History in the same sentence and you've opened a Pandora's box. At one level, you can look back a millenium and catch glimpses of great kings and temples. At a level up, you get the French and Indochine and the remnant buildings of Empire.
As early as 1858, Vietnamese Catholics fleeing persecution by the Vietnamese Emperor Tu Duc settled in Cambodia. Above this, you have the war in Vietnam and the boat people, some of whom went to America but many came by boat up the river to Cambodia.
Today, a quick ferry ride from the dock at the back of the Himawari Hotel takes you to a living remnant of these Vietnamese. Although the influence of Vietnam can be found everywhere along the Mekong into the heart of Phnom Penh, the most important ghetto is around Arei Ksart.
Below is the boat that goes back and forth to the island.
Another vignette of history found in Areiksart are the two statues of Mary and the Child Jesus found buried in the riverbed by Vietnamese fishermen. These statues must have come from some of the Churches in Phnom Penh when they were destroyed during the Khmer Rouge reign. There used to be a huge Cathedral in Phnom Penh built in 1951. A Phnom Penh Post article details the story of this Cathedral.
Others said that they may have come from the boats of the fleeing Vietnamese Catholics. More interesting is how they found the statues. You can read this on the board in Our Queen of Peace Church in Areiksart.
Here is one of the statues in the Queen of Peace Church.