Five Parts Per Trillion

The garden smelled like it was about to happen before a single drop fell.

Maya stopped walking. Just stopped, mid-step, with one foot still in the air.

"There," she said.

Soren looked up from the bean stakes he was counting for Mrs. Okafor, who had gone inside to answer her phone and left them to finish the tally. "There what?"

"That smell."

He sniffed. Warm dirt. The faint sweetness of the tomato vines. Something underneath those things that he couldn't name. "It smells like outside."

"It smells like rain," Maya said. "Except it hasn't rained."

Soren checked the sky. Solid white-gray from edge to edge, the kind of clouds that meant business, but nothing falling. The soil under the raised beds was pale and cracked. He crouched and pressed his thumb into it. Dust. Hard as a plate.

"Could be the humidity," he said. "Air pressure changing."

"Maybe." Maya was already kneeling next to the nearest bed, her nose two inches from the dirt.

"You're sniffing the ground," Soren said.

"I know."

"Okay."

He crouched beside her and tried it. The smell was stronger here. Not stronger like a candle burning in a room. Stronger like something very small had just been let out of something very small. He didn't have better words for it than that.

"It's coming from in there," Maya said, pointing at the soil. Not at the sky. Not at the incoming weather. At the dirt itself.

Soren pulled out his notebook and wrote: smell from soil not rain. stronger near surface. what's in there.

"Worms?" he said.

"Worms don't smell like this."

"Fungus?"

"Maybe. But it happens every time before rain. Every time. That's not a coincidence."

Mrs. Okafor came back out, phone still in hand, already talking to someone else. She waved at them and pointed at the bean stakes, meaning: keep counting. Soren waved back. They kept counting. But his mind was still in the soil.

That evening, because Soren needed the mechanism before he could believe the conclusion, he looked it up. He read one article and then read it again more slowly. Then he went to find Maya.

She was sitting on her back porch watching the clouds move.

"Bacteria," he said.

Maya turned around. "What?"

"The smell. It's bacteria. There are these bacteria that live in soil, they're called Streptomyces, and when conditions change -- when humidity rises, when rain is coming -- they release a compound. It's called geosmin." He said the word carefully. Gee-oz-min. "And that's the smell. That's exactly the smell."

Maya stared at him. "Bacteria make that smell."

"They've been living in that dirt the whole time. Mrs. Okafor's whole garden. Under our feet all summer."

"And the rain triggers it?"

"The change does. The pressure, the humidity. Something tips them into releasing it."

Maya was quiet for a moment. "So the smell isn't rain. The smell is the soil reacting to the fact that rain is coming."

"Yeah."

"The ground is announcing it."

Soren opened his notebook to the page he'd been writing on. "There's more."

"Tell me."

"Five parts per trillion." He tapped the page. "That's the concentration where humans can smell geosmin. Five molecules out of every trillion molecules in the air. That's how little it takes. Scientists tested it. We can detect geosmin better than almost any instrument. Better than almost any other animal. We are one of the most sensitive geosmin detectors known."

The clouds were turning the particular gray that meant the rain wasn't waiting much longer.

Maya held very still. Soren recognized what that meant. She was running it somewhere inside her head.

"That's not an accident," she said finally.

"That's what I can't figure out," Soren said. "Why would we be that sensitive to exactly that molecule?"

"Because it matters," Maya said. "It mattered. To people before us. Finding water. Knowing where water was about to be."

"Maybe," Soren said. "That's one idea. Nobody knows for certain."

"Nobody knows."

"No."

She turned and looked at him. "We're built to smell something that bacteria make in the ground to announce rain, and nobody knows why we're built that way."

"That's accurate."

The first drop hit the porch rail. Then three more. Then the sound on the leaves changed and became continuous.

The smell arrived fully then, not the faint early edge of it but the whole thing, opened like a door. Five parts per trillion became a thousand parts per trillion and the garden filled with it. Soren breathed in and felt his chest do something strange, something that wasn't sadness and wasn't happiness but was like standing at the edge of a very large room in the dark and understanding that the room went on farther than any light could reach.

Maya stood up and walked off the porch into the rain. She didn't run for cover. She stood in it with her face up.

"There are people who study this," Soren said from the dry part, near the door. "Chemists. Microbiologists. Evolutionary biologists. They all want to know the same thing and none of them have figured it out yet."

Maya lowered her face and looked at him through the rain. Water was running down her forehead and she didn't wipe it away.

"Then it's still open," she said.

"Completely open."

She reached her hand out flat, palm up, and watched the drops hit it one at a time.

Soren stepped off the porch and stood next to her. He tipped his face up the same way she had.

Five parts per trillion. The bacteria were still working under their feet, under the softening soil, under the whole soaked garden. He could smell them and they had no idea he existed.

The rain came down harder, and Maya kept her palm out, and neither of them moved.