They really are for making hydrogen.

Iraqi Trailers Said to Make Hydrogen, Not Biological Arms: " Engineering experts from the Defense Intelligence Agency have come to believe that the most likely use for two mysterious trailers found in Iraq was to produce hydrogen for weather balloons rather than to make biological weapons, government officials say."
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A Defense Department official said the alternative views expressed by members of the engineering team, not yet spelled out in a formal report, had prompted the Defense Intelligence Agency to "pursue additional information" to determine whether those Iraqi claims were indeed accurate.