Found in the deep Underworld, below the mapped tunnels, in chambers that predate the city's founding. Mnemic Clay is a dense, cool, slate-gray material with the consistency of wet clay. It is unremarkable to look at. It is not unremarkable to touch.

Mnemic Clay retains the shape of whatever last contacted it — but not immediately. Press your hand into it and remove it: the surface is smooth. Over the next several hours, a perfect impression of your hand will form, accurate to the fingerprint level, as if the clay is remembering what touched it and slowly reconstructing the memory.

This property alone would make it a curiosity. What makes it something else is that Mnemic Clay sometimes forms shapes that nothing pressed into it. A researcher studying a sample left it on her desk overnight. In the morning, it had formed a detailed model of the house she grew up in — a house in Lagos that was demolished when she was nine. She had never described it to anyone. She had no photographs of it. The clay reproduced it from a perspective she recognized as the view from her childhood bedroom window.

Mnemic Clay does not read minds in any measurable way. BCI scans during contact show no data transfer, no electromagnetic interaction, no neural activity change. It simply knows what you remember, and sometimes it shows you.

Small quantities are traded in the Underworld at extreme prices. The Cartographers have documented 14 known deposits. They do not share the locations.