Archaeologists began searching Tuesday for unmarked mass graves containing hundreds of unbaptized babies and infants buried by the Catholic Church on the edge of a Belfast cemetery.

Unlike many other Christian churches, the Catholic Church teaches that baptism is essential for a soul to enter heaven and therefore the ritual must take place as near to birth as possible. For decades, newborns and infants who die before baptism were deemed ineligible for salvation and were not buried on consecrated, or holy, ground.

Individual priests in Belfast began loosening that restriction in the 1980s as families demanded the right to bury their youngest loved ones in marked family plots.

"We're coming out of what we can only regard as a mistaken theology of a hundred years ago," said the Rev. John McManus, a Belfast priest who has been working with local families demanding that their children's resting place be mapped, marked and preserved. "People have been carrying the grief and burden of losing a child for decades. It's important we get this right."

Although Catholics have long believed that children who die without being baptized are with original sin and thus excluded from heaven, the Church has no formal doctrine on the matter. Theologians long taught that such children enjoy an eternal state of perfect natural happiness, a state commonly called limbo, but without being in communion with God.

In 2007, Pope Benedict XVI reversed centuries of that teaching by approving a report by the International Theological Commission, a Vatican advisory panel, that said there were "serious" grounds to hope that children who die without being baptized can go to heaven.

The report stressed that none of its findings should be taken as diminishing the need for parents to baptize infants.

The records of Milltown Cemetery in overwhelmingly Catholic west Belfast indicate that hundreds of unbaptized newborns and infants were interred in unmarked mass graves on the western edge of the cemetery — land that the church sold nearly a decade ago to a nature reserve.

Since October, mothers and other relatives of a few dozen of the dead infants have protested at the cemetery fence.

The families,under the umbrella name Relatives for the Milltown Babies, are demanding that church leaders apologize, identify the grave sites, and return the land to the cemetery with new grave markings.

Their pressure spurred Catholic authorities to ask the government's Environment Department to intervene with archaeological experts from Queen's University of Belfast. Church leaders have declined to make any formal public response pending the archaeologists' findings.

The search is expected to cover an acre of woods and grassland within the nature reserve, called the Bog Meadows, and run through Friday. The archaeologists — some of whom previously helped search for hidden graves of Irish Republican Army victims, an issue in Northern Ireland's peace process — are using ground-penetrating radar to identify grave sites and will not disinter any remains.

The Ulster Wildlife Trust — which in 2001 acquired the supposedly unused land from Milltown Cemetery in a 999-year lease deal — says it will transfer any land found to contain graves back to the cemetery.

The Northern Ireland Environment Agency's assistant director of built heritage, John O'Keeffe, cautioned families of the dead that the search would not lead to any positive identifications of specific remains.

"We will not be able to provide any greater degree of certainty than already exists about the precise location of specific individuals thought to have been buried here," said O'Keeffe, who asked media to stay away from the search operation "given the sensitivities around this, particularly for the families involved."