A dog can never tell you what she knows from the smells of the world, but you know, watching her, that you know almost nothing.
~ Mary Oliver, Dog Songs
Paintings: Andrew Wyeth
… Dogs' sense of smell overpowers our own by orders of magnitude—it's 10,000 to 100,000 times as acute, scientists say... "If you make the analogy to vision, what you and I can see at a third of a mile, a dog could see more than 3,000 miles away and still see as well."
… Dogs can detect some odors in parts per trillion. What does that mean in terms we might understand? … while we might notice if our coffee has had a teaspoon of sugar added to it, a dog could detect a teaspoon of sugar in a million gallons of water, or two Olympic-sized pools worth.
… Experts have reported incredible true stories about the acuteness of dogs' sense of smell. There's the drug-sniffing dog that "found" a plastic container packed with 35 pounds of marijuana submerged in gasoline within a gas tank. There's the black lab stray from the streets of Seattle that can detect floating orca scat from up to a mile away across the choppy waters of Puget Sound. There's the cancer-sniffing dog that "insisted" on melanoma in a spot on a patient's skin that doctors had already pronounced cancer-free; a subsequent biopsy confirmed melanoma in a small fraction of the cells. And so on...
From “Dog's Dazzling Sense of Smell” – NOVA