Why is a Soviet ship lying on the bottom of the Marlborough Sounds? Produced by Justin Gregory.

Why is a Soviet ship lying on the bottom of the Marlborough Sounds?

"Like a 55-storey building at the bottom of the ocean."

Port Gore Bay in the Marlborough Sounds is a beautiful place - from the shore. But head out onto the water even just a short distance and you can soon see another side. The water here is deep, very cold and murky. Turbulence from Cook Strait churns up the bay, making it rough to swim in and lowering visibility to nearly zero.

Only the very good and the very brave go diving here. After tying up to a nondescript buoy, divers slip into the water and clutch tightly to a taut, shuddering rope that leads them down through the depths. At about 15 meters below the surface they come upon what looks like the bottom of the bay. It's only when you touch it do you realise that this long, curved mound was once a ship.

That ship was the Mikhail Lermontov, once the pride of the Soviet Union's fleet of cruise ships. The Lermontov has been lying on the bottom of Port Gore bay for more than 30 years. Very few people get to see it now and even then, they don't see much of it.

Derek Grzelewski has seen as much of the ship as he cares to. Derek is a writer and a filmmaker, the author of the book 'Going to Extremes', with a chapter about diving on the wreck of the Mikhail Lermontov. It's an eerie, uncanny experience, to explore a dead ship Derek which describes as "like a 55 storey building lying on its side at the bottom of the ocean".

'Everything just recedes into the gloom,' says Derek. 'You're in this void, in this dome of what you can see and outside of that it's just darkness.

'It's basically the stuff of your nightmares.'

But what is a Soviet ship doing lying on the bottom of the Marlborough Sounds? Well, it's a funny old tale and it doesn't have a happy ending or even really an ending at all.

The port town of Picton has seen a lot of ships but not many like the Mikhail Lermontov. At 150 metres long and twelve decks high, she was far bigger than anything else around her when she docked in the early hours of Saturday 15 February 1986. Under Captain Vladislav Vorobyov this elegant white cruise ship had crossed the Tasman from Sydney with more than 700 passengers and crew on board, on what advertising claimed would be the "experience of a lifetime". They'd visited Auckland, Tauranga and Wellington and were sailing to Milford Sound before heading home. The pilot who'd guided her into Picton would later take her out again; local harbourmaster Captain Don Jamison, a man with decades of experience in these waters…

Go to this episode on rnz.co.nz for more details