Why U.S. natural gas output keeps rising as prices sink.

U.S. natural gas prices last week plunged to a 30-month low, crossing below $2 per million British thermal units (mmBtu) for the second time this year, even as some producers have cut drilling to stave off further convulsions.

Since the start of the year, U.S. gas futures have collapsed by about 50 percent, a record drop for a quarter, on rising output and mostly mild weather so far this winter that kept heating demand low and allowed utilities to leave more gas in storage than usual.

But...