A dead lander recorded the Moon ringing for forty-one minutes. One note in that recording should not exist.

On an August morning in 2026, a spent rocket stage strikes the far side of the Moon. The impact is routine, predicted to the second, and watched by no one.

Eleven kilometres away, a lander lying on its side is still listening.

Mariam Osei has eight days of battery and no funding. Meret Kessler has built a career dismantling claims exactly like hers. Together, they begin ruling out every ordinary explanation — instrument error, bad timing, corrupted data, wishful thinking.

Then the ordinary explanations run out.

The impossible note points toward something on the Moon placed with extraordinary care, unimaginably old, and still functioning. Confirming it will require data no agency wants to fund, access treaty law was never written to permit, and a signature no one has the authority to give.

The Solar System was surveyed a long time ago. Humanity is extraordinarily late to notice.

Rigorous, patient, and quietly enormous, Witness Mark is hard science fiction about the discovery that changes everything — and the bureaucracy capable of burying it.