Winter in the Great Smoky Mountains drove most life into shelter, whether living people or walkers.
But for Dr. Jenner, holed up in his laboratory, it was an excellent window for experimentation.
Research on the walker attractant had never stopped.
All winter, while everyone else was running around for survival supplies and warmth, he had buried himself almost completely in the precision scientific equipment brought back from Sentinel Station.
Using the blood samples Calista provided regularly, Jenner carried out complex separation, purification, and synthesis.
He tried isolating different components, then added chemical stabilizers and volatilization promoters to create several pale amber and faintly reddish liquids with different concentrations and properties.
Dr. Jenner labeled the reagents as Series A, high-concentration serum, Series B, medium-concentration synthetic attractant, Series C, low-concentration pheromone mimetic, and so on.
Then the walker experiments could begin.
The biggest difficulty was probably obtaining test subjects in winter.
Defense Force teams took risks in frozen valleys far from the base, using special traps and sturdy mesh cages to capture more than a dozen walkers whose movements had become extremely sluggish in the low temperature.
They were secretly kept at an outdoor testing site in the Great Smoky Mountains, chained separately to the surrounding trees with thick iron chains. Their mouths and arms were tightly bound, only to be released when needed for experiments.
The outdoor testing site was beside a hidden abandoned mine pit in the mountains. It was surrounded by mountains on all sides, relatively isolated, and could effectively block off smell and sound. Even if something went wrong, it would be easy to control and clean up.
Carver and Turner had chosen this location.
Back when Rock Fortress was still Twilight Manor, they had used this place to dispose of the steward and his son after they came to the manor to cause trouble.
At the time, Carver and the others did not yet know that people would turn after death. They had simply locked the bodies in a car and driven it into the mine pit.
When they returned later to choose the site, the mine pit had already been partly flooded by rain, and the lower halves of the steward and his son had long since rotted away.
The steward's upper body was pressed against the car window, clawing at the car body.
His son had already crawled out through the window and was floating face-up in the foul water, his mouth opening and closing as he rasped.
Carver thought they were too noisy, so he put them down from a distance with two shots.
Later, based on Carver's suggestion, Rickson established Dr. Jenner's outdoor experimental facility here.
The first test took place after a light snowfall.
Jenner placed a hand-built automatic sprayer in the center, filled with B-2 reagent, medium concentration.
At radii of 50 meters, 100 meters, 200 meters, and 300 meters from the spray point, he set up simple alarm devices made from thin wire and bells.
A walker was brought there and secured in a sturdy cage about 400 meters downwind from the spray point. The cage was fitted with a strong electronic lock.
The experiment began. The reagent formed a fine aerosol through the sprayer and slowly spread through the cold, dry air.
Jenner watched the walker closely through binoculars.
For the first few minutes, the walker showed no reaction.
Just as he began to suspect the concentration was too low, the walker, which had been almost stiff from the cold, began to stir restlessly.
Its rotting head lifted. Its empty eye sockets "looked" toward the spray point, and a muffled huffing sound came from its throat.
It began to struggle, trying to break free from the cage, its strength clearly increasing.
Dr. Jenner signaled the Defense Force members assisting with the experiment to open the cage.
The Defense Force guard operated it remotely and unlocked the cage.
The walker came out, dragging its rotting limbs forward a few steps, then began wandering aimlessly.
The bell alarms remained silent.
The effective attraction radius was less than 100 meters, worse than ordinary human scent.
Jenner recorded the first failure in his notebook: "B-2, low-temperature experiment, insufficient diffusion, attraction radius <100m, limited effect."
For the second test, he used the more volatile A-1 reagent, high-concentration serum.
This time, Jenner improved the method by placing small amounts of raw meat at several key distance points as "beacons," then observed whether the walkers would be attracted to the meat and remain at the beacon locations.
He also extended the simple alarm range to 1,000 meters, testing along the way whether it could draw wild walkers out of the woods.
The result was alarming.
Less than three minutes after the reagent was sprayed, the walker placed 350 meters downwind showed extreme aggression and slammed frantically into the cage.
Even more troublesome was what happened about fifteen minutes after the experiment began. A wild walker that had been wandering in a nearby valley staggered into the edge of the testing site, as if it had heard some invisible call, and headed straight for the spray point.
Although it was silently killed by the Defense Force about 600 meters from the spray point, the data was clear.
Jenner recorded: "A-1, high concentration, strong volatility, effective attraction radius in low-temperature environment approximately greater than 1000m, can attract non-target individuals, risk too high."
In the following tests, he tried slow-release gels and repeatedly adjusted the reagent concentration and the conditions under different weather.
The testing process was full of uncertainty.
Sometimes the reagent was almost ineffective. Other times, it brought unexpected trouble.
One test even nearly caused a small group of walkers to deviate from the planned route. Fortunately, a nearby scout patrol noticed early and guided them away.
Finally, on a windless morning at minus ten degrees Celsius, Jenner found the critical balance point.
He used C-4 reagent, a low-concentration pheromone mimetic that had been improved many times.
It sacrificed some instant attraction in exchange for a steadier, longer-lasting slow release, and its diffusion pattern in low temperatures was more controllable.
The experimental target was a walker locked 500 meters from the spray point.
At the same time, concealed Defense Force observers were stationed at distance markers in every direction.
Time passed second by second.
Ten minutes later, the walker began turning toward the spray point.
Twenty minutes later, it began trying to move in that direction in a sustained, rhythmic way.
At the same time, an observer reported that a bell set about 800 meters downwind had been triggered once, indicating that a wild walker had been attracted into that range.
But the wild walker did not continue approaching. At that distance, the attraction seemed to have reached a balance with the interference from the environment.
Jenner's heart pounded violently.
He checked the data again and again, then adjusted the model.
"This is it..." he murmured, a trace of feverish excitement finally appearing on his frost-reddened face.
Jenner immediately packed up the equipment, ordered the site cleaned up, and returned to the base as quickly as he could.
Back in the laboratory, Dr. Jenner called Dr. Evans over. The two spent half a day organizing the data, drawing charts, and recalculating everything repeatedly.
The final result clearly pointed to one conclusion. Under the current specific conditions of severe cold and a light breeze, the optimized C-4 attractant could create an effective attraction range with a radius of roughly 500 to 750 meters.
Within that range, walkers would clearly be drawn in and continue moving toward the source of the attractant.
Beyond 800 meters, the effect declined sharply and was heavily disrupted by wind direction, terrain, and the walkers' own condition.
This range far exceeded Jenner's initial expectations, and also far surpassed any known attraction range based on ordinary human scent or sound.