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The deepest hole humans have ever dug reaches 12 kilometers below the ground of Russia’s Kola Peninsula. Although we now have a spacecraft on its way to Pluto--about six billion kilometers away from the sun--we still cannot send a probe into the deep earth. For practical purposes, then, the center of the planet, which lies 6,380 kilometers below us, is farther away than the edge of our solar system. In fact, Pluto was discovered in 1930, and the existence of the earth’s inner core was not established--using seismological data--until six years later.

Still, earth scientists have gained a surprising amount of insight about our planet. We know it is roughly structured like an onion, with the core, mantle and crust forming concentric layers. The mantle constitutes about 85 percent of the earth’s volume, and its slow stirring drives the geologic cataclysms of the crust. This middle domain is mainly a mix of silicon, iron, oxygen, magnesium--each of which appears in roughly the same concentrations throughout the mantle--plus smaller amounts of other elements. But depending on the depth, these elements combine into different types of minerals. Thus, the mantle is itself divided into concentric layers, with different minerals predominating at different depths.

Most of us don’t think twice about getting behind the wheel even for short hops to pick up some milk. And that’s not just because cars are convenient, or because we’re lazy. According to Andrew Furman of Ryerson University in Toronto [ http://bit.ly/dr76ot ], it’s because in many places in North America it’s just not that nice to walk. But if cities and suburbs put more effort into building better pedestrian routes, he says more people might leave their SUVs at home. [More]

Scientific American magazine Editor in Chief Mariette DiChristina and news editor Philip Yam join podcast host Steve Mirsky (pictured) to talk about the cover story of the June issue of the magazine, "12 Events That Will Change Everything". [More]

Lawmakers have floated a proposal to fast-track electric cars , but some in the clean-car field are worried that the wheels may fall off.

Last week, House and Senate legislators released bipartisan plans to speed up the deployment of electric vehicles. In each plan, the centerpiece was a "targeted deployment" approach: Rather than offer the same incentives nationwide, the government would award federal funds to the regions that come up with the best blueprints for rolling out tens of thousands of plug-in cars.

For at least 50 years Alaska's Bristol Bay has been one of the most valuable fisheries in the U.S. On average, fishermen net about 25 million sockeye salmon annually in the bay's chilly waters. In 2009 the catch was worth more than $120 million. [More]

Editor's Note: William Gilly , a professor of cell and developmental biology and marine and organismal biology at Stanford University, is traveling with a group of students on board the Don José in the Sea of Cortez. The team is monitoring and tracking Humboldt squid and sperm whales in their watery habitats. This is the group's 10th and final blog post. [More]