The retired salt caverns, aquifers, and fuel depots that hold Europe's stockpiles of natural gas have never been so empty at this point in winter.

Just four months after Amos Hochstein, the U.S. envoy for energy security, said Europe wasn't doing enough to prepare for the dark and cold season ahead, the continent is grappling with a supply crunch that's caused benchmark gas prices to more than quadruple from last year's levels, squeezing businesses and households. The crisis has left the European Union at the mercy of the weather and Russian President Vladimir Putin's wiles, both notoriously difficult to predict.