No one knows how large they can grow - but giant, blind, glow-in-the-dark earthworms are wriggling a couple of metres beneath the surface around Auckland.

This 75cm-long specimen was a surprise discovery for the owner of the Ti Point Reptile Park in Warkworth, who came across the worm on his morning rounds of the grounds.

"It was just lying on the ground alongside the track in the bush, for some reason it had come out of its burrow," Ivan Borich said.

Specimens of Spenceriella gigantea are rarely seen live above ground.

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In 40 years running the park, Mr Borich had never seen one.

"We've always known about them. In all these years we'd always liked to have seen one, but when they're that long and the clay ground's that hard it'd be impossible to get one out in one piece."

Entomologist Ruud Kleinpaste said that was because the worms were safe in the earth and knew it would be dangerous to venture out.

"If you come above the ground and you are 1.4 metres and, say, an inch or an inch and a half thick, you are a serious breakfast. And secondly, you can't crawl away quickly, you're not a snake. The fastest you can move is in your tunnels below the ground."

But this week's torrential rain would have driven this specimen out of its tunnels.

"When it rains really hard, their burrows can run full of water ... they don't want to drown, they don't have gills, they need oxygen to run through their skin. Also with downpours, you get slips which crush their tunnels," Mr Kleinpaste said.

These giant worms were much more common than people realised, he said.

This week's discovery was a star at the Reptile Park yesterday, and staff were keeping it in a bucket of soil.

Mr Borich said they wanted to hold onto it until nightfall so they could see whether it glowed in the dark before setting it free.

Mr Kleinpaste said it was a little-known fact that earthworms could glow.

"It's been reported that we have native earthworms that are fluorescent - why that would be I have no idea, because these things don't even have eyes."

SPENCERIELLA GIGANTEA
* Also known as North Auckland worm.
* Thought to be New Zealand's largest.
* No one knows how long it can grow.
* Can glow in the dark.
* Burrows through the earth in a network of tunnels.
* Has twice as many hairs as normal earthworms and feels "unshaven".