Local caver Peter Crossley takes us on a tour of an underground lava cave in the backyard of Annie Jacob's property.

Where: Landscape Rd, Mt Eden

What: Lava cave

Age: 30,000 years old, discovered before European settlers came to New Zealand.

After walking through Annie and Sean Jacob's backyard there's no doubt the Mt Eden residents take the prize for the most unique garden feature.

The pair have owned their own lava cave for the past eight years after coveting the cave, which was originally on their neighbour's property, for years prior.

Annie Jacob said the family jumped at the chance to have their own cave when the neighbour's property came up for sale.

Thousands of people have been into the cave since, thanks to the Jacob family opening their backyard to the Mercy Hospice Heroic Gardens tour and visits from family and friends.

"It's something that needs to be seen."

Jacob said the family used it in summer to keep cool and had hosted parties down there on occasion.

The cave was a unique part of the area's history, she said.

"It's just part of Mt Eden, part of Three Kings, and it's just very cool to have something like that in your backyard, it should be preserved and looked after."

Speleologist Peter Crossley said the 30,000-year-old cave was formed when the lava flow from the Three Kings eruption came down the mountain and flowed through the valley.

The surface of the lava flow solidified and the liquid inside drained away, leaving "incandescent hot dripping rock", he said.

Crossley had found about 250 lava caves in the Auckland region but only about half were accessible and of those only about a dozen were as big as the Landscape Road cave.

When Maori came to New Zealand and discovered the caves they used the entrances as burial grounds.

When Europeans came the bones were moved by Maori to protect them from "suburbia encroaching".

Home owners in surrounding properties were then known to use the lava caves for mushroom farms because of the dampness and constant temperature, wine cellars, air raid shelters and, unfortunately, rubbish tips.