A pair of German physicists claim to have broken the speed of light - an achievement that would undermine our entire understanding of space and time.

According to Einstein's special theory of relativity, it would require an infinite amount of energy to propel an object at more than 186,000 miles per second.

However, Dr Gunter Nimtz and Dr Alfons Stahlhofen, of the University of Koblenz, say they may have breached a key tenet of that theory.

The pair say they have conducted an experiment in which microwave photons - energetic packets of light - travelled "instantaneously" between a pair of prisms that had been moved up to 3ft apart.

The most distant galaxy cluster yet has been discovered by combining data from NASA's Chandra X-ray Observatory and optical and infrared telescopes. The cluster is located about 10.2 billion light years away, and is observed as it was when the Universe was only about a quarter of its present age.

The galaxy cluster, known as JKCS041, beats the previous record holder by about a billion light years. Galaxy clusters are the largest gravitationally bound objects in the Universe. Finding such a large structure at this very early epoch can reveal important information about how the Universe evolved at this crucial stage.

A settlement that long ago sank into the Mediterranean Sea has been identified as the world's oldest underwater town. Pavlopetri, off the southern coast of the Pelopennese in Greece, has been dated to around 3000 BC.

Although Pavlopetri was found in 1967, the Greek government has just announced that 5000-year-old pottery fragments have been recovered from the town, forcing a rethink of when it was first occupied.

Moreover, the government has also revealed that a further 9000 square metres of buildings, streets, and graves - plus what looks like a large ceremonial building called a megaron - have been discovered. This suggests that Pavlopetri may have been an important trading port, and provides new clues about how Neolithic people lived.

"You can find scattered huts or Palaeolithic caves [on the sea bed] which are much older, but not towns with streets, and rows of houses sharing common walls," says Nic Flemming of the National Oceanography Centre in Southampton, UK, who first discovered Pavlopetri in the 1960s and dated it to around 1500 BC.

"What we've got here is something that's 2000 or even 3000 years older than most of the submerged cities that have been studied. And its uniqueness is not just its age, but the fact that it was used as a port."

If you have been puzzled by Mona Lisa's smile - how she's radiant one moment and serious the next instant - then your worries are over. It happens because our eyes are sending mixed signals to the brain about her smile.

Different cells in the retina transmit different categories of information or "channels" to the brain. These channels encode data about an object's size, clarity, brightness and location in the visual field.

"Sometimes one channel wins over the other, and you see the smile, sometimes others take over and you don't see the smile," says Luis Martinez Otero, a neuroscientist at Institute of Neuroscience in Alicante, Spain, who conducted the study along with Diego Alonso Pablos.

This isn't the first time scientists have deconstructed Leonardo da Vinci's masterpiece. In 2000, Margaret Livingstone, a neuroscientist at Harvard Medical School with a side interest in art history, showed that Mona Lisa's smile is more apparent in peripheral vision than dead-centre, or foveal, vision. And in 2005, an American team suggested that random noise in the path from retina to visual cortex determines whether we see a smile or not.

The dangles on a cable hanging from an eight-storey-high tower. Suspended in a harness with his back to the ground, he sees only the face of the man above, who controls the winch that is lifting him to the top of the tower like a bundle of cargo. And then it happens. The cable suddenly unclips and he plummets towards the concrete below.

Panic sets in, but he's been given an assignment and so, fighting his fear of death, he stares at the instrument strapped to his wrist, before falling into the sweet embrace of a safety net. A team of scientists will spend weeks studying the results.

The experiment was extreme, certainly, but the neuroscientist behind the study, David Eagleman at Baylor College of Medicine in Houston, Texas, is no Dr Strangelove. When we look back at scary situations, they often seem to have occurred in slow motion. Eagleman wanted to know whether the brain's clock actually accelerates - making external events appear abnormally slow in comparison with the brain's workings - or whether the slo-mo is just an artefact of our memory.

It's just one of many mysteries concerning how we experience time that we are only now beginning to crack. "Time," says Eagleman, "is much weirder than we think it is."

By understanding the mechanisms of our brain's clock, Eagleman and others hope to learn ways of temporarily resetting its tick. This might improve our mental speed and reaction times. What's more, since time is crucial to our perception of causality, a faulty internal clock might also explain the delusions suffered by people with schizophrenia.

Ever since Arthur Eddington travelled to the island of Príncipe off Africa to measure starlight bending around the sun during a 1919 eclipse, evidence for Einstein's theory of general relativity has only become stronger. Could it now be that starlight from distant galaxies is illuminating cracks in the theory's foundation?

Everything from the concept of the black hole to GPS timing owes a debt to the theory of general relativity, which describes how gravity arises from the geometry of space and time. The sun's gravitational field, for instance, bends starlight passing nearby because its mass is warping the surrounding space-time. This theory has held up to precision tests in the solar system and beyond, and has explained everything from the odd orbit of Mercury to the way pairs of neutron stars perform their pas de deux.

Yet it is still not clear how well general relativity holds up over cosmic scales, at distances much larger than the span of single galaxies. Now the first, tentative hint of a deviation from general relativity has been found. While the evidence is far from watertight, if confirmed by bigger surveys, it may indicate either that Einstein's theory is incomplete, or else that dark energy, the stuff thought to be accelerating the expansion of the universe, is much weirder than we thought (see Not dark energy, dark fluid below).

The analysis of starlight data by cosmologist Rachel Bean of Cornell University in Ithaca, New York, has generated quite a stir. Shortly after the paper was published on the pre-print physics archive, prominent physicist Sean Carroll of the California Institute of Technology in Pasadena praised Bean's research. "This is serious work by a respected cosmologist," he wrote on his blog Cosmic Variance. "Either the result is wrong, and we should be working hard to find out why, or it's right, and we're on the cusp of a revolution."

The sun's million-degree outer atmosphere is the last place you would expect to find rain, yet a form of it does occur there. The stuff could help explain why the sun's outer atmosphere, or corona, is much hotter than closer in.

Coronal rain is made of dense knots thousands of kilometres across consisting of relatively cold gas, at tens or hundreds of thousands of degrees C, which pours down towards the sun's visible surface from the outer atmosphere at speeds exceeding 100 kilometres per second. "There's just this constant rain of these blobs that seem to be coming down from high up," says Judy Karpen of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

When Spanish conquistadors seized the Inca emperor Atawalpa in 1532, they demanded an enormous ransom of silver and gold. For weeks, llama trains carried tons of gold and silver statues, cups, and other objects to the Europeans, who then ordered them melted down to ingots for transport back to Spain. Such an enormous stash suggests that the Andean people knew sophisticated metallurgy, but there has been little evidence to support this. Now a team of geologists and archaeologists have found clues that these indigenous people refined gold with mercury amalgamation, an important metallurgical technique that is still in use today.

Scientists have discovered a site containing the most extensive evidence seen so far in Mexico for the Clovis culture. The find extends the range of America's oldest identifiable culture, which roamed North America about 13,000 years ago.

The bed of artefacts in the state of Sonora in northwest Mexico also includes the bones of an extinct cousin of the mastodon called a gomphothere. The beast was probably hunted and killed by the Clovis people, known for their distinctive spear points, who mysteriously disappeared within about 500 years of leaving their first archeological traces.

A team of Spanish and Portuguese researchers has carried out molecular genetic analysis of the Y chromosome (transmitted only by males) of the aboriginal population of the Canary Islands to determine their origin and the extent to which they have survived in the current population. The results suggest a North African origin for these paternal lineages which, unlike maternal lineages, have declined to the point of being practically replaced today by European lineages.

Researchers from the University of La Laguna (ULL), the Institute of Pathology and Molecular Immunology from the University of Porto (Portugal) and the Institute of Legal Medicine from the University of Santiago de Compostela (USC) have studied the Y chromosome from human dental remains from the Canary Islands, and have determined the origin and evolution of paternal lineages from the pre-Hispanic era to the present day. To date, only mitochondrial DNA has been studied, which merely reflects the evolution of maternal lineages.