麻豆传媒

IN THE early hours of 10 June 1886, the ground beneath Mount Tarawera on New Zealand鈥檚 North Island began to shake. After an intensifying swarm of earthquakes, the volcano erupted from three domes, spewing hot gases and rocks into the air. Surveyor Henry Roche observed the inferno from his camp nearby, describing 鈥渢he striking spectacle of a dark, flat-topped mountain more than a mile long, red hot along its crest, and surmounted by a wall of fire 1500 feet high鈥�.

Finally, a fourth explosive eruption occurred directly below Lake Rotomahana, one of several waterbodies surrounding the volcano. The lake and the sediments on its bed were blasted hundreds of metres into the sky with a force so great that it totally reshaped the surrounding landscape. And what a landscape. On opposite slopes of the lake, water cascaded down the hillsides into pink and white rock pools. Local Maori called them their taonga, or treasure. Others saw them as the eighth wonder of the world. On the day of the eruption, they disappeared without trace.

Over time, the so-called Pink and White Terraces have largely faded into obscurity, although New Zealanders still cherish the memory of them. But a few years ago, a chance find hinted that they might have survived after all, at least in part. Then, historical records came to light putting that discovery in doubt. It suggested that researchers had been looking in the wrong place and that the terraces might have had an even more intriguing fate.

The Pink and White Terraces were actually two gigantic deposits of silica, formed when the mineral precipitated out of hydrothermal fluids trickling down the hillside from hot springs above. It鈥檚 not clear how long this process took, says . 鈥淢ost likely hundreds, if not thousands of years.鈥� The Pink Terrace, on the lake鈥檚 western shore, was called . Its tinge came from traces of impurities, including gold. The Maori called the , meaning 鈥渢attooed rock鈥�.

Former glory

Similar terraces can be seen at Hierapolis near Pamukkale in Turkey, and in Yellowstone National Park in the US 鈥� although, being made from limestone rather than silica, they don鈥檛 shimmer as beautifully. Like them, New Zealand鈥檚 terraces held naturally heated pools. Local resident Willy Bennett, one of the last people to see the terraces, described the spectacle in : 鈥淗ot mineral water swirled about in two large pools and then overflowed over a series of basins, or hollowed terraces, that stood out in ever-increasing semi-circles descending to the shore of the lake.鈥� In the 19th century, these were popular with tourists 鈥� many of whom bathed nude. They would first go for a dip at the lower levels and work their way up to the hotter baths closer to the springs.

has studied numerous tourist accounts, historical paintings and photographs of the terraces. His research builds a picture of beautiful blue basins glowing in the sunlight, glistening waterfalls and 鈥渃rystal formations which gave a chandelier-like look鈥�. 鈥淭ruly they were a thing of beauty, and a joy forever for the few who saw them,鈥� recalled Bennett.

Then, when Bennett was 12, Mount Tarawera erupted. The last blast was especially destructive. It was an unusual kind, called a hydrothermal eruption, which doesn鈥檛 release magma. These occur when water trapped underground at high temperatures and pressures is explosively released. It is this final blast that is thought to have blown the terraces to smithereens. It also buried nearby Maori villages under metres of rock, mud, ash and other debris, killing at least 120 people.

In the weeks that followed, several groups made the treacherous journey to the site to see the devastation for themselves. Among the first to arrive was a group of journalists, including . Two days after the eruption, they climbed to the summit of a nearby hill to survey the scene. , Leys describes how Lake Rotomahana had completely vanished: 鈥渘ow, instead of a splendid sheet of water, there was鈥� a huge crater, belching out showers of mud and stones from innumerable yawning mouths鈥�.

This crater slowly filled with water from rivers and streams to form a new Lake Rotomahana, which still exists today. It is 88 metres deeper than the original and five times its area, covering 8 square kilometres.

And that was how things remained for 125 years, until de Ronde decided to map the lake. He wanted to find out what happens to a huge hydrothermal system when it is submerged beneath 100 metres of water. To do this, in 2011 he teamed up with researchers from Woods Hole Oceanographic Institution in Massachusetts. Using high-resolution sonar imaging techniques, the researchers found huge cracks in the lakebed, probably caused by the 1886 eruption. They also found 750 plumes of volcanic gas bubbling up from the floor 鈥� evidence that plenty of geothermal activity remains. In fact, the team calculated that Lake Rotomahana has the largest heat flow 鈥� the movement of energy from Earth鈥檚 interior to its surface 鈥� of any lake in the world. The estimated total heat output from the lake is as much as 130 megawatts: enough to power more than 100,000 households.

The research was considered so significant it was published in a . It was the first time anyone had found a hydrothermal system that had survived being submerged. But that wasn鈥檛 all the researchers found. Within their 4 square kilometre scan of the lake floor, they spied a patch unlike anything any of them had ever seen before. It appeared to be a thin, rocky outcrop about 60 metres below the surface. Intrigued, they dropped a camera down to get a better look. The gently sloping outcrop was almost entirely covered in a thick layer of sediment, but peeping out in a few places were patches of white rock with a pinkish hue. In February 2011, they . The following year, while going over their scans of the lake, they found a similar-shaped feature. .

鈥淲as it possible that part of one or other or both of these features had survived 鈥� perhaps buried rather than destroyed?鈥� says Ronald Keam, a physicist and amateur historian at the University of Auckland. 鈥淭his was definitely a part of the fascination that inspired my own intrigue with the region.鈥�

Keam wasn鈥檛 just intrigued, he realised there was something he could do to help. 鈥淩otomahana had not been officially surveyed before the volcanic eruption, so that no one knew exactly where the terraces had been situated,鈥� he says. To pin them down, he went in search of pre-1886 photographs from the area and then used them to map their original locations. His calculations gave , relative to a landmark that partially survived the eruption, a ridge that once carried the White Terrace on its western flank.

In 2014, de Ronde and his team returned to the lake. Using Keam鈥檚 mapping, they repositioned their camera a kilometre north-east of where the Pink Terrace fragment had been found. Bingo! There was a similar formation: a lump of pale rock, resembling dripping candle wax. The formation was within a 6 per cent error of Keam鈥檚 estimated location for the White Terrace, which de Ronde describes as 鈥減retty remarkable, considering the errors in estimating distances using photogrammetry鈥�. They were confident their previous discovery was part of the Pink Terrace too, despite it being some 20 metres deeper than Keam had predicted.

It was unclear whether these were fragments blasted loose by the eruption or large pieces of intact terrace. However, based on their underwater photographs, the researchers concluded that around 10 per cent of the terraces survived and now lie .

This seemed like the end of the story. Then, last year, Bunn reported a potentially explosive discovery. He had been unconvinced by de Ronde鈥檚 interpretation of his finds. 鈥淗ad the terraces been under water or within that lake boundary of the new Lake Rotomahana, then one would have to say they were lost because they would have gone skyward in the volcanic eruption,鈥� he says.

鈥淚f Bunn is right, the terraces aren鈥檛 under the lake at all, but lie nearby, buried in ash鈥�

The new evidence offered an alternative story. Sascha Nolden, a research librarian at the National Library of New Zealand, had uncovered previously unknown field survey diaries and maps created by , a German-Austrian geologist who, in 1859, was the first professional scientist to study Mount Tarawera. When Bunn and Nolden read one of his diaries, they concluded that . They were not at the blast site at all, but in the surrounding hillsides.

How can this mistake have arisen? Bunn claims that most maps of the pre-eruption lake and terraces perpetuated a simple error. He believes it originated in a map by German cartographer August Heinrich Petermann published in 1864. This seems to have moved Lake Rotomahana and changed its shape from Hochstetter鈥檚 original, possibly to make room for a large map legend. Future cartographers then copied the mistake, says Bunn.

If, as Hochstetter鈥檚 diary suggests, the terraces were adjacent to the blast, then instead of being blown apart, they would have been covered by ash and other debris. This is unlikely to have done much damage. 鈥淭he material they were made from is chemically very stable,鈥� says Bunn. What鈥檚 more, hydrothermal eruptions are much cooler than those that produce magma, spewing out debris at temperatures between 150 and 250掳C. That鈥檚 not hot enough to melt or significantly deform silica. If Bunn is right, the terraces aren鈥檛 under Lake Rotomahana at all, but instead lie nearby, almost wholly intact, albeit buried under metres of soil and ash.

Hidden treasure

De Ronde finds Bunn鈥檚 conclusions difficult to accept. 鈥淚t was exciting that Bunn and colleague located the diary of Hochstetter that contained the actual measurements he took in the field,鈥� he says. But he cautions that Bunn鈥檚 findings need to be reconciled with the rest of the available evidence. Keam is sceptical too, and says the new information must be considered alongside other historical records and photographs before drawing firm conclusions. In particular, he points out that Hochstetter鈥檚 1859 diary 鈥渄irectly contradicts鈥� what is shown in the maps he made in 1863 and 1865. However, Bunn says these maps were also subject to Petermann鈥檚 error.

The obvious way to resolve the argument is to go and look. Bunn plans to use ground-penetrating radar at the sites where he believes the terraces lie hidden. 鈥淪ilica formations underground reflect ground-penetrating radar very well,鈥� he says. 鈥淚f the terraces are there, GPR will show them.鈥� The next step would be to drill down and take a core sample. These cores could be compared with known samples of the Pink Terrace, which have a distinctive chemical signature. 鈥淚f we can find a Pink Terrace core sample with antimony and arsenic sulphide, and perhaps a touch of gold, I鈥檒l be prepared to claim absolute evidence that we鈥檝e found the Pink Terrace,鈥� says Bunn. He suggests the terraces should then be excavated and restored by archaeologists.

One potential problem with this plan is technical. De Ronde says the rugged terrain around Lake Rotomahana will make it hard to conduct a GPR search. The terraces would also be undetectable were they to be more than 6 metres down. If Bunn does find them, there is another issue. Any plans for the terraces need to consider the wishes of the local Maori, who are stewards of Lake Rotomahana and the surrounding region. Bunn is discussing his idea of restoring the terraces with them. But de Ronde suspects the Maori would not be keen to see the terraces excavated and restored. 鈥淗aving worked closely with them, it is my understanding that they want to let things stay the way they are,鈥� he says. 鈥淭hey are happy that they stay 鈥榲eiled鈥�.鈥�

, Alan Skipworth, chairman of the Tuhourangi Tribal Authority, said that before proceeding with the search, the Maori want to be confident it is in the right place. To that end, they have requested an independent review of Hochstetter鈥檚 documents. 鈥淥ur focus at this stage is on trying to pinpoint an exact location, nothing more,鈥� he said.

Even if the terraces are found, excavated and restored, they will never assume their former glory. The geysers that once filled them with warm water are now inactive. Nevertheless, the terraces themselves would be a sight to behold. 鈥淚f we could bring back an eighth wonder of the world, that would be absolutely stunning,鈥� says Bunn.

This article appeared in print under the headline 鈥淟ost and found鈥�