The stunning terraces of Lake Rotomahana were obliterated by a volcanic eruption in 1886, but geologists have now found traces of them hidden at the bottom of the lake

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In the early hours of 10 June 1886, Mount Tarawera, a volcano on the North Island of New Zealand, erupted with astonishing force. The explosions may have been heard as far afield as Christchurch, more than 400 miles (640km) to the south-west.

The eruption killed 120 people, most of them Maoris – native New Zealanders – living in small villages in the surrounding countryside. But it is not just because of its high death toll that the Tarawera eruption is firmly lodged in the collective memory of New Zealanders. Most people also remember the eruption because it robbed the island nation of a treasured natural wonder: the Pink and White Terraces of Lake Rotomahana.

The terraces were the two largest formations of silica sinter – a fine-grained version of quartz – ever known to have existed on Earth. They were located on opposite shores of Lake Rotomahana, situated six miles (10km) to the south-west of Mount Tarawera. And they were extraordinarily beautiful.

The terraces are sometimes even described as the eighth wonder of the world. One was a brilliant white colour, while the other, due to an unknown chemical impurity, was tinged a light shade of pink. Either would easily have constituted a geological marvel on its own merits. But to have two such wonders within sight of each other, and in complementary colours, made the Pink and White Terraces greater than the sum of their parts.

Watercolour paintings from the 19th Century – particularly those of Charles Blomfield – offer a sense of their magnificence.

By the time the eruption died down, sometime around dawn, Lake Rotomahana had vanished

"It's hard to describe to non-New Zealanders what the terraces mean to us," says Cornel de Ronde, a geologist at GNS Science, a state research institute in New Zealand.

Which explains why the events on that cool June morning have become so infamous.

Shortly after 3am, about an hour after Mount Tarawera rumbled to life, the eruption spread to Lake Rotomahana, and a cluster of volcanic craters opened up on the lake floor.

In 1886 Lake Rotomahana was not visible from any of the surrounding villages, meaning most of the locals could only hear the eruption. But by chance, one person, who happened to be spending the night in the countryside seven miles (11km) to the east, had an unobstructed view of Lake Rotomahana.

The Pink and White Terraces had been destroyed, or at best permanently entombed

Two days later the man, Henry Burt, told a journalist that during the eruption the lake had resembled "a huge boiling cauldron, bubbling [over] in all directions".

By the time the eruption died down, sometime around dawn, Lake Rotomahana had vanished. The water had literally been blasted into the air, combining with the erupted volcanic ash to form a muddy sludge that buried the surrounding countryside up to 46ft (14m) deep.

The newly-formed volcanic craters that had blown apart the floor of Lake Rotomahana were still belching out mud and rocks two days later, when the first expedition party arrived to survey the damage.

Not only was the lake gone, but so too it seemed were the Pink and White Terraces. They were nowhere to be seen; the area where they had once stood was caked in volcanic mud. And the white chunks of sinter which some people found mixed in with the volcanic debris did not inspire hope.

The shocking state of Lake Rotomahana and its surroundings after the 1886 eruption led New Zealanders to believe that the Pink and White Terraces had been destroyed, or at best permanently entombed.

Secretly, some of the researchers were hoping to resolve the mystery that had surrounded the terraces

In the months after the eruption, Lake Rotomahana grew back. Water from surrounding streams began to flow into in the volcanic craters which had blown apart the old lake, gradually forming a new lake four times deeper and covering an area approximately five times the size of the original.

By the end of 1886, the area where the Pink and White Terraces had once stood was under tens of metres of lake water. It seemed like they had been lost for good.

It would be 128 years before the world learned their true fate.

Between 2011 and 2014, scientists from GNS Science led a series of expeditions to map and study the floor of Lake Rotomahana. The expedition was not related to the Pink and White Terraces directly. The team's goal was to discover how the 1886 eruption had affected the geothermal system that had formed the terraces.

But secretly, some of the researchers were hoping to resolve the mystery that had surrounded the terraces since that dark day in 1886. In their more optimistic moments, some of the scientists may even have hoped to find physical remains.

"I did quietly wonder what we might find in the areas where the terraces had once sat," says de Ronde, who led some of the lake expeditions.

But that was dreaming. In the meantime there was important science to be done.

The team was able to build up the sharpest image of the floor of Lake Rotomahana ever made

One of the researchers' many aims whilst at Lake Rotomahana was to construct high-resolution images of the lake floor, a task which was carried out by de Ronde and his colleagues from New Zealand and the United States. They hoped that by mapping the lake floor in unprecedented detail, they would somehow be able to discover the fate of the lake's geothermal system.

The team obtained the images using, amongst other gadgets, two high-resolution ‘side-scan' sonar instruments. They strapped these onto a pair of torpedo-shaped AUVs (autonomous underwater vehicles), which they then remotely manoeuvred through the lake, travelling 31ft (10m) above its floor, mapping the lake floor as they went.

The team performed the sonar survey in February 2011. In order to cover the almost 3.5 square miles (9 sq km) of lake floor, the researchers directed the AUVs along a series of parallel, linear paths – each of which imaged a long but narrow strip of the lake floor, almost as if mowing a lawn.

By combining many such scans – which totalled approximately 186 miles (300km) of scan lines – the team was able to build up the sharpest image of the floor of Lake Rotomahana ever made.

The sonar images are not what you would see with your eyes down there (which, thanks to the gloom and sediment, would not be much anyway). Instead, the images were in black and white – the dark areas represented sediment, while the brighter parts generally represented rocky material or gases.

The picture of the lake floor that emerged on the sonar scans was dominated by sediment, as the team had expected. But there were plenty of interesting features on the sonar images too.

The team saw what appeared to be a long, thin rocky outcrop

Some showed what looked like cracks – geological faults – crossing sections of the lake floor. de Ronde thinks these were probably formed during the 1886 eruption.

Elsewhere on the sonar map of the lake floor, the team saw ghostly-looking clouds of bubbles rising from small craters on the lake floor, proof that volcanic gases are still emanating from the floor of Lake Rotomahana today.

De Ronde was used to seeing features like these on sonar scans. He had carried out similar analyses of volcanoes on the seafloor. But on one of the sonar images, taken in the north-western corner of Lake Rotomahana, de Ronde and his team saw something they had never seen before, on a sonar image or on any kind of image.

Jutting above the lake floor, the team saw what appeared to be a long, thin rocky outcrop, stretching for a horizontal distance of around 197ft (60m).

The scientists did not know what they had found. The feature clearly was not a fault: faults are sharply delineated, whereas the mystery outcrop had a misshapen, lumpy appearance.

But it was the location of this strange feature that really grabbed the team's attention. They were in the area of the lake floor where the Pink Terraces had once stood.

With the eye of faith they even looked a little pinkish

The researchers knew that the lumpy outcrop they had discovered could not possibly be the whole of the Pink Terraces; it was only a fraction of their size. But could it be a fragment? This thought alone was enough to get the scientists' pulses racing.

The team wanted a photograph right away. Sonar was one thing, but to determine whether the feature might actually be a section of the Pink Terraces, they needed to examine it visually.

They had to wait three years. When the researchers first spotted the possible terrace remains in 2011, they had been too caught up with completing the rest of their surveys to deploy their deep-water camera. But when the team returned to Lake Rotomahana for their next round of surveys, in February 2014, they positioned their boat above the possible terrace remains and dropped down the camera.

Most of the photographs the team took showed sediment and not much else. But to be safe, they took thousands. And on two of those photographs, de Ronde and his colleagues found something that definitely was not sediment. It also made them smile.

The photos showed a rounded, rocky-looking outcrop, sloping gently to one side. And the parts that were not draped in sediment had a bright, pale appearance. With the eye of faith they even looked a little pinkish.

This photographic evidence, coupled with the fitting location of the rocks on the lake floor, was enough to confirm it: de Ronde and his colleagues were looking at a section of the Pink Terraces. They had suddenly resurrected a natural wonder.

But the best was still to come. Buoyed by their re-discovery of the Pink Terraces, the team re-deployed the aquatic camera approximately 0.6 miles (1km) to the north-east: the relative location, in the pre-1886 landscape, of the White Terraces.

Amazingly, these photographs showed much the same thing. In the location where the White Terraces had stood before the 1886 eruption, the camera showed a large lump of pale rock.

If anything, this second outcrop looked even more terrace-like than the first. The whitish rock seemed to have the same vertical, columnar texture that is apparent in pre-eruption photographs of the White Terraces. De Ronde likens its texture to candle wax.

The team had chalked up another discovery, and Lake Rotomahana had suddenly regained pieces of both of its famous terraces. They had been amputated, drowned and buried by over a century of sediment, but somehow, incredibly, they had partially survived the eruption of 1886.

The photographs and sonar images showing the terrace remnants are published in the Journal of Volcanology and Geothermal Research.

They certainly were just fragments of the terraces – only 10% of the Pink Terraces seem to have survived, and even less of the White Terraces – but that fact almost added to the rarity of the discovery for the researchers.

Technically speaking, the terraces had always been a sidebar to the group's mission – a romantic distraction, but nothing of scientific substance. But in other ways, the terraces meant a great deal to the scientists – and to New Zealanders more generally.

Burt would probably have been amazed that anything at all could have survived

"I suppose it's a bit like Americans finding evidence for a long-lost Grand Canyon," says de Ronde.

At the end of their 2014 expedition, the researchers left Lake Rotomahana for the last time. The group had achieved their scientific goals: they had established that the geothermal system at the lake was still active, and they had produced a bathymetric map of the lake floor that was 400 times sharper than any other.

But they had also helped shed new light on the events of 10 June 1886. From his viewpoint seven miles away, Henry Burt – the only eyewitness – probably saw relatively little. The volumes of ash already coming from Mount Tarawera, and the volcanic mud that was about to issue from Lake Rotomahana itself, would soon have blocked his view.

But the new discoveries made at Lake Rotomahana provide a second witness to what happened that night. The fact that only fragments of the Pink and White terraces remain today strongly suggests that the majority of these old wonders were indeed destroyed by the eruption, probably soon after it began.

But Burt would probably have been amazed that anything at all could have survived the catastrophe he witnessed.

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