An informal name for hilly and lineated terrain found on Mercury and the Moon. On Mercury, weird terrain is found on the opposite side of the planet to the large Caloris Basin, and is thought to have formed when the seismic waves from the Caloris impact converged, a process known as seismic focusing. Weird terrain on the Moon, not so clearly defined as on Mercury, occurs in two places, one directly opposite the centre of the Imbrium Basin, the other opposite the centre of the Orientale Basin.