Key Points:

A mass tomb for six Catholic bishops has been built under the floor of the newly restored St Patrick's Cathedral.

Church authorities decided to plan for the future when they undertook extensive ground works beneath the cathedral in the past two years.

The entire floor of the cathedral and much of the exterior grounds was dug up in the massive structural strengthening job headed by Fletcher Construction.

And while such extensive works were on, the opportunity was taken to add the extra touch with the mass crypt.

Cathedral administrator Father Bernard Kiely said it was a time-honoured tradition to bury bishops under European cathedrals.

So a deep pit was burrowed out beneath the sanctuary, between the new, carved altar and the just-uncovered medieval stained glass rose window on the back wall.

"There were such major excavations, so we wanted to be able to take advantage of that - and we had the space to do it," Father Bernard said.

The large, deep chasm was dug metres down with mechanical equipment, a thick concrete pad was laid as its floor, the entire area was lined with concrete blocks, it was sealed off, then the sanctuary was built over the top.

In Catholic tradition, priests are usually the ones who walk over that area around the altar. "The area is only accessible through the floor," Father Bernard said.

But just whether Patrick Dunn, the Catholic bishop of the Auckland diocese, will be laid in that crypt when he passes away is not something Father Bernard wants to speculate about.

One bishop already rests there.

Bishop George Lenihan, who served until 1910, is buried under the sanctuary in front of the altar in a tomb topped with an ornate brass crucifix, marble and slate. That tomb had been hidden for decades until Fletcher Construction demolished the old sanctuary and rebuilt it.

As the ground was dug up around him, the bishop - encased in his cement tomb - was at one stage lying just above ground level in a large excavated cavity.

When the grave was uncovered, a new mystery emerged: soon after the unearthing, a curious construction worker used his cellphone to take a picture of the structure housing the relics. But the image showed something more: the clear outline of a human skeleton on top of the grave. Rumours flew around the site about the picture, which soon made its way on to some computers.

No one at the cathedral could explain the mystery, how the image arrived on the worker's cellphone or what it might mean.

That tomb is now covered by the sanctuary. A new cross has been installed to mark the grave.

Ringing in the changes

Some of the changes to St Patrick's Cathedral, at 43 Wyndham St, are:

* A new slate tile roof

* New foundations and restored north lawn

* Earthquake strengthened walls

* New baptismal font

* New pulpit near sanctuary

* Repaired stained glass windows

* Strengthened bell tower

Mystery of lost nuns deepens

Mystery shrouds the whereabouts of nuns' graves concealed beneath Auckland's historic Catholic cathedral, in spite of an extensive search.

The $12.8 million conservation and restoration of St Patrick's, which culminates in a grand reopening this weekend, involved exhaustive excavation and archaeological works both inside and outside the cathedral.

But it did nothing to solve the riddle surrounding missing Sisters of Mercy, thought to be lying in an unmarked cemetery alongside the cathedral. Two years ago, ground-penetrating radar tests strongly indicated the nuns' graves were in the area. Tests were made before the cathedral was shut for a major restoration and upgrade which has strengthened the structure against earthquakes and restored many historic features.

When the work was starting in late 2005, Dr Simon Best, the archaeologist supervising historic excavation, said operators doing the radar tests and using a magnetometer had found anomalies in the area which they had interpreted to be possible graves. But after two years of digging and searching, church authorities are none the wiser.

Kevin Sherlock, the cathedral's business manager, said this week that objects picked up by the tests were probably not graves.

"There were several pits that Simon excavated in the Wyndham St lawn area that yielded school slates, porcelain figures and pottery pieces from the St Patrick's school and orphanage that the Mercy sisters ran in the 1850-1860s on the cathedral site," he said.

These were of interest, but not entirely what was expected.

Father Bernard Kiely, the cathedral administrator, said the nuns could well be under the actual cathedral wall.

Between 1854 and 1863, 10 nuns were buried in the Wyndham St area, under the gardens of their convent which once stood alongside the cathedral.

When the church was expanded in 1882, some bodies were exhumed and were shifted to St Mary's Convent in Ponsonby's New St. But at least two were left behind. Just who stayed remains a mystery due to a mismatch between names on a plaque installed on a cathedral wall and markings on the Ponsonby graves.