The upper layers of a well-designed securitisation vehicle should be all but impervious to loss.

ECONOMIST: Securitisation

That would have made the Atlantic's upper layers less dense, so less liable to sink.

ECONOMIST: The science

The largest earthquakes in the world occur close to plate margins, areas of the Earth's upper layers that are being rammed together or pulled apart.

BBC: Q&A: UK's small-scale earthquakes

And the hot plasma can also heat up and expand the upper layers of the atmosphere, causing satellites in low orbits to encounter more air resistance and lose altitude.

ECONOMIST: Hot stuff

And now the Arctic's upper layers are getting less dense, for several reasons: melting Arctic glaciers, rising surface-water temperatures, increased precipitation and an absence of salt concentrations resulting from sea-ice formation.

ECONOMIST: The science

Weather forecasters have said that recent rising temperatures and strong winds in the area have made it more dangerous as upper layers of snow are more likely to become detached, Italian media report.

BBC: Italy avalanche kills two in Fiemme Valley

The researchers suggest that while the upper layers of Messel Lake were hospitable enough to allow turtles to live, its deeper layers may have been a toxic death trap, perhaps from the buildup of volcanic gases or poisonous decaying organic matter.

MSN: Fossilized turtles' sex act frozen in time - Technology & science - Science - LiveScience | NBC News