THE Pyramid of the Sun may fall apart. One side is dry while another side is wet, which could lead to the pyramid’s collapse unless a fix can be found.

Between the 1st and 7th centuries, Mexico’s Pyramid of the Sun was at the heart of the largest city in the Americas. Now known as Teotihuacan, the lost city had a population of more than 125,000, making it one of the biggest in the world. The pyramid itself is among the largest on the planet. Its exterior is covered with 3 million tonnes of volcanic rock, but the interior is a mound of earth.

From 2010 to 2013, of the National Autonomous University of Mexico (UNAM) in Mexico City and colleagues studied the interior of the pyramid using muons. These sub-atomic particles can pass through most materials, but are deflected when they hit denser ones. That means more muons reach the other side if an object has an internal cavity, filled with less dense air. So by tracking the paths of muons through the pyramid, Menchaca could create a 3D representation of its insides.

To do this, his team placed muon detectors under the centre of the pyramid, in a tunnel that runs beneath its base. The muons originate in space as cosmic rays, which break up into smaller particles when they pass through Earth’s atmosphere.

The team was looking for internal chambers, but none was apparent. In contrast, the nearby Pyramid of the Moon contains royal tombs.

Instead they found a problem: the density of the earth in the pyramid is at least 20 per cent lower on one side than the other. “The pyramid is at risk of collapsing if something isn’t done,” says Menchaca. He presented his results at a conference on Teotihuacan at UNAM last month.

“The pyramid is not going to collapse tomorrow, but it is at risk of collapsing if something is not done”

Menchaca believes the difference is caused by the south side drying out. He compares the pyramid to a sandcastle on a beach. “I can use slightly moist sand to make a sandcastle,” he says. “If I leave it exposed to the sun and touch it when it is dry, then it crumbles.”

The pyramid is “not going to collapse tomorrow”, Menchaca says. “But it is the same phenomenon we observe in the subsoil of Mexico City.” Mexico’s capital is built on a dried-out lake, and every year as water is extracted from aquifers beneath it.

Opinion is divided on how to save the pyramid. Menchaca suggests wetting the dry side.

But the real problem may be excess water, not dryness, says Alejandro Sarabia, the site director at Teotihuacan. “Decades ago, cement was added between the covering stones. This added stability and hindered the growth of vegetation,” he says. “On the other hand, it prevents evaporation of damp created by water seeping through gaps.”

Sarabia says archaeologists are now replacing the cement with more suitable materials like river sand.

This article appeared in print under the headline “Mexico’s great pyramid under threat of collapse”

The warship sank on 2 May 1982 during a short war over the control of the Falkland Islands, which Argentines call the Malvinas. The wreck is a war grave for 323 Argentine soldiers and sailors and lies about 185 kilometres from Tierra del Fuego. The relatives of the dead approved the mission on the understanding that bodies would not be disturbed.

The US National Geographic Society (NGS) is conducting the search and plans to produce a television documentary. It will also place a commemorative plaque on the wreck and retrieve some symbolic elements of the ship, to be placed in the Argentine Navy Museum.

But finding the vessel will not be easy. It lies in waters about 4200 metres deep, where sunlight does not penetrate. Therefore, side-scan sonar imaging will be used to pinpoint the location of the General Belgrano. This is expected to take between seven and 10 days, assisted by the relatively flat ocean floor in the area.

However, the frequently stormy waters of the South Atlantic also present a challenge. The expected “weather window” is only 10 days – after that strong winds and rough seas are likely to end the mission. The NGS rates expedition’s chance of success as “better than 50 per cent, depending on the weather”.

Spotlights and cameras

If the wreck is found, a crewless mini-submarine will be launched, also guided by sonar. The submarine’s spotlights and cameras will enable it to search for scattered debris and determine whether the ship broke in two, as some experts believe. A robot arm will be used to collect material for return to the surface.

The NGS has rented an Argentine oceanographic ship for the expedition. Pedro Luis Galazi, who was second in command of the General Belgrano will be on board. The captain, Hector Bonzo, it too ill for the voyage.

An Argentine naval vessel will accompany the mission, carrying another 20 survivors and relatives of those who died. Many of these bitterly opposed the NGS inviting two former crewmembers from HMS Conqueror, the nuclear submarine that sank the General Belgrano.

The sinking of the Argentine cruiser was extremely controversial, and the expedition’s discoveries could re-ignite the argument. The General Belgrano is believed to have been attacked whilst outside, and sailing away from, a 200-mile exclusion zone imposed by British forces.

The region’s worst weather is expected in August and the sea freezes up to 1000 kilometres from land in September. “Getting out is going to be harder than getting in” said the icebreaker’s captain Hector Tavecchia.

The icebreaker, the Almirante Irizar, is breaking a path for the freed ship to follow. However, the sea ice is reportedly up to four metres thick and progress so far has been limited to 4km/h (2 knots).

The Madalena Oldendorf had been trapped since early June, after it had picked up scientists from the Russian base of Novolazarevskaya. The icebreaker arrived at the rescue site on Friday, just a week before the German ship would have run out of food and fuel.

But the transfer of fuel and supplies was initially delayed by bitter weather. Temperatures dipped to -20 °C and winds gusting at over 100 km/h brought the chill factor much lower.

Late departure

Normally all ships leave Antarctica in April but the Russians had waited for the ice to harden so that they could offload heavy equipment. On its way towards Cape Town the ship encountered impassable ice and drifted back about 320 kilometres to the edge of the permanent sea ice, just west of the Greenwich meridian.

All but 17 of the 108 scientists and crew on board had already been rescued by a South African polar supply ship, the SA Agulhas, which left Cape Town on June 16.

Its early arrival meant it encountered ice only a few centimetres thick and had no trouble getting close enough to dispatch the ship’s two helicopters. These had just a few hours of daylight in which to work.

The 176 Argentines on the icebreaker know that they too could become trapped – a winter rescue in Antarctica has never previously been attempted. However, should the worst happen, the Almirante Irizar has enough fuel on board to circumnavigate the globe and supplies to last a year.