The Cave was a machine in Oxford, Will's world which the Dark Matter Research Unit used to communicate with what they called Shadow-particles. Mary Malone (the director of the Unit) showed it to Lyra Silvertongue when she came to visit. Lyra immediately recognised it as similar to her alethiometer and easily communicated with the computer.
Oliver Payne experimented with the Cave by getting a piece of ivory, this didn't react or hold any Shadows, but an ivory chess piece reacted. He then got some fossil skulls and saw how far back the effect went - this was around 30/40 000 years prior when humans began to walk the planet. The Cave could react with anything that had been associated with human workmanship.
Workings and usage
To connect to a human properly,the user had to go through a two processes. Gel was put on the skin of the human to help with the electrical contact before six wires were stuck to the head. To use the Cave, a human had to empty their mind and look at the screen.
Lyra and the Cave
Before Lyra could use the Cave, she proved to Mary Malone that she knew what she was doing by using her alethiometer to tell Mary her life story about being a nun. Mary, at first, didn't think Lyra would be able to use the Cave, not having any experience with it and quoted a line of text by Keats to tell her about the state of mind she had to be in. When Lyra first used the Cave, she wowed Mary by making floods of colours go onto the screen before thinking of the Cave as her alethiometer and making it show pictures. This told her that the computer could be coded so that it showed words.
When Mary Malone first used the Cave and it responded with words, she began to find out more about Dust and how they made up angels. The replies were almost instantaneous and it told Mary to play the serpent in the new Adam and Eve tale. It also warned her the machine must be destroyed to leave no evidence of the fact she had been able to talk to Dust.
After the Cave told her it must be destroyed, Mary wasted no time in taking down anything to do with talking to the particles, this included removing her I Ching poster as well as clearing and smashing the disk with the rest of the components before leaving her laboratory forever.