Among the newest guardians of 2.5 million people in the San Francisco Bay Area, and the water they drink, is a tiny group, on duty 24 hours a day inside a well-secured treatment plant.
In a tank, to be exact, are eight fish, all bluegill.
"We're able to get a jump on monitoring the slightest change in our water quality, from things that would cause no harm to humans to things that may cause death," says Susan Leal with the Public Utilities Commission.
The fish react to even traces of poisons by abruptly changing their movements. A computer system watches for that behavior, ready to sound the alarm.
"Fish, unlike any other man-made sensor, have a very broadband or wide detection range," says Bill Lawler with Intelligent Automation Corp., which helped design the San Francisco system. "They can detect a number of toxic chemicals. They don't need calibration. And they're never wrong."
Since 9/11, there's been an explosion in demand for another kind of highly sensitive detector, this one in the noses of dogs. They can be trained to sniff out thousands of kinds of explosives, at such minute levels that even sensitive electronic detectors can't find them.
Eric Rizza, a police officer from Everett, Mass., wants to be a dog handler to help safeguard his city's huge liquefied natural gas port. What has surprised him about the training he's had so far?
"How far we've progressed in a week," says Rizza. "I mean, it seems like the dogs are ahead of us at this point."
In Front Royal, Va., the Bureau of Alcohol, Tobacco and Firearms (ATF) teaches dogs to find weapons or explosives but to ignore other potent smells, from food to perfume. When they find a potential hazard — like a handgun — the dogs sit. ATF uses real-world experience to update the explosives in its training, recently adding the mix of chemicals used in the July 2005 London subway bombings.
"When they get intelligence out in the field, they contact us, and we can change our training to meet the requirements to the threat," says ATF dog trainer Shawn Crawford.
It's a threat that's bringing hundreds of eager new animal recruits to the front lines.