For this clue to be hidden, there was a Janus and a Tomas agent who teamed up in order to hide the clue. Emperor Puyi, a Chinese emperor, wanted to hide the clue on top of Mt. Everest. However, knowing he could not make the trip himself, he asked the mountaineer, and Tomas agent, George Mallory to do it for him. Mallory did as he was asked but died on his trip down from the top.
Emperor Puyi created a silk cloth that read the following;
That which you seek, you hold in your hand,
Fixed forever in birth,
Where the Earth meets the sky
The first line stated that the clue was in the person's hands. Saying that the cloth was the clue, and since the cloth was made out of silk, silk was the clue. However, the last line was referring to Mt. Everest, where the Janus serum was hidden by George Mallory years ago. It was decided to be placed there because it would freeze and be preserved until it was found.
Silk is a highly valued fabric that is made from the secreted substance of silk worms. Silk worms are the larvae of the Domestic Silkmoth and they can only produce silk as larvae.