Scientists are baffled by a mysterious "dead zone" in the Gulf of Mexico off southern Florida, an area normally rife with fish but described by fishermen now as fouled by murky waters and barren of marine life.

And like any good mystery, investigators haven't ruled out any culprit in their search for the cause of the zone, which spans from Marathon Key to Naples.

Samples researchers took Tuesday were in water they said was unusually dark.

"Goodness gracious," said Erich Bartels, a biologist with Mote, "seven feet of water and you can't see the bottom."

That was in water that is usually postcard turquoise with clear water in the shallow parts. This was mostly blackish green and pea green at smaller depths.

Fish spotter pilots were the first to discover the black water in January. Though fishermen didn't find dead fish in its wake, they report an abysmal season for those waters and unusual behavior in the few fish they did find.

While the images from the private company Orbimage's SeaWiFS and a NASA satellite show that the water might be coming from the Shark River, not all the pictures are consistent with that possibility, Muller-Karger said.