It's a pumpkin!
Called a pumpkin for obvious reasons, this device is used to cache water for the use of firefighters combatting blazes in areas inaccessible to ground vehicles. The pumpkin holds about 500 gallons, and a helicopter equipped with a bucket on the end of a long tether can fly over and fill the bucket by dipping it into the water. It then goes to the fire and dumps its load.
Thanks go to Mark Holloway of Beatty, Nevada, for photographing this pumpkin near Ballarat, California.