When a thick lava flow cools it contracts vertically but cracks perpendicular to its directional flow with remarkable geometric regularity - in most cases forming a regular grid of remarkable hexagonal extrusions that almost appear to be made by man. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns. 
One of the most famous such examples is the Giant's Causeway on the coast of Ireland (shown above) though the largest and most widely recognized would be Devil's Tower in Wyoming. Basalt also forms different but equally fascinating ways when eruptions are exposed to air or water. 
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