Puzzler...

Last week's puzzler was this: what caused the “erratic boulders” that are found all over the country of Estonia? Most of you (13 of 17) got it right: the erratic boulders were left by the retreating glaciers. The boulders were originally scraped from the pink granite bedrock exposed in northern and southern Finland, and then carried hundreds of miles south to Estonia. As the glaciers melted, the boulders gradually descended until finally they were left on the bare limestone ground of Estonia. The Vikings carved runes into some of these boulders, and examples of such carved boulders can be seen in many locations in Estonia. In addition, the local cultures paid attention to many of them (especially the larger ones), and many of the boulders have footpaths to them, and ladders to climb them, and these are maintained to this day. A popular roadmap made by an Estonia company has the location of hundreds of these boulders marked, and in my travels around Estonia I have visited dozens of them, just for fun.

This week's puzzler tests your knowledge about the Earth's atmosphere. You most likely know that the atmospheric pressure is highest at sea level (well, actually, at those points on the Earth that are below sea level), and that the pressure falls as you go to higher and higher elevations. But at what altitude does the atmospheric pressure fall to half the pressure at sea level?

No fair Googling until after you answer!