The SCP is a patch of land that rises and falls in a slow, steady rhythm, as if breathing. Attempts to dig into the earth cause discoloration, like a bruise, and bleeding. 'Hurting' the patch of land too much caused minor seismic activity, so further attempts to dig are discouraged. Analysis of the 'blood' reveals it to be from some unknown creature.
Deep underground chambers near the SCP have the sound of deep breathing echoing throughout them, and if the walls of the chambers are earthen (like in caverns, mineshafts, etc.) they display the same properties as the SCP when attempts to dig into them are made.
The patch of land has a history of moving. Disappearing from one spot and appearing in another, teleporting, and taking the 'breathing' effect with it. Recently, the breathing effect is less pronounced deep underground, but people in a town where the object once settled have complained of deep breathing sounds in their basements.