A MASS grave thought to contain the remains of babies and young children has been discovered at a former Catholic orphanage in Ireland where almost 800 perished in deaths that were never documented.

The Mother and Baby Homes Commission said excavations since November at the site of the former Bon Secours Mother and Baby Home in Tuam, County Galway, had made disturbing discoveries.

11 Workers at the former Bon Secours Mother and Baby Home in Tuam, County Galway Credit: EPA

11 An old picture of Bon Secours Mother and Baby Home

11 Local historian Catherine Corless tracked down death certificates for nearly 800 children. None of the children pictured here are among them Credit: www.adoptionrightsalliance.com

11 The dead ranged from 35 weeks to three years old and were buried mainly in the 1950s. None of the children pictured here are among them Credit: www.adoptionrightsalliance.com

Investigators have found an underground structure divided into 20 chambers containing "significant quantities of human remains".

DNA analysis of selected remains confirmed the ages of the dead ranged from 35 weeks to three years old and were buried mainly in the 1950s.

The home closed in 1961.

11 Excavators found children's remains inside a neighbouring connected structure. Pictured, nuns and children at the home in days gone by Credit: www.adoptionrightsalliance.com

11 The home closed in 1961. Pictured is a plaque at the shrine in Tuam Credit: Getty Images

Today's findings offer the first pieces of evidence after decades of suspicions that the vast majority of children who died at the home had been buried on the site in unmarked graves during the period of high child mortality rates across Ireland.

The probe was launched in 2014 after local historian Catherine Corless tracked down death certificates for nearly 800 children who had died as residents of the facility.

However she could find only one child's burial record.

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Corless said: "Everything pointed to this area being a mass grave."

Katherine Zappone, the government's commissioner for children, said the findings were "sad and disturbing."

She added: "We will honour their memory and make sure that we take the right actions now to treat their remains appropriately."

11 800 children died at the facility but Catherine Corless, pictured with her husband Aidan, could find only one child's burial record Credit: andynewman6@hotmail.com

11 There are suspicions that the vast majority of children who died at the home had been buried in unmarked graves Credit: rayryanphotos@gmail.com

11 Local historians say that 'everything pointed to this area being a mass grave' Credit: EPA

Investigators found that a decommissioned septic tank had been "filled with rubble and debris and then covered with top soil" and did not appear to contain remains.

However excavators found children's remains inside a neighbouring connected structure that may have been used to contain sewage or waste water.

11 The Bon Secours Sisters order of nuns ran the home until its closure Credit: rayryanphotos@gmail.com

11 The probe was first launched in 2014, but the scale of the mass graves have only been revealed today Credit: EPA

The investigators, who are examining the treatment of children at a long-closed network of 14 Mother and Baby Homes, said they still were trying to identify "who was responsible for the disposal of human remains in this way".

The Bon Secours Sisters order of nuns, which ran the home until its closure, said in a statement that all its records, including of potential burials, had been handed to state authorities in 1961. It pledged to cooperate with the continuing investigation.

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