The Congregation is a natural amphitheater in the deep Underworld, approximately 300 meters in diameter, located at a depth estimated between 400 and 500 meters — well below the deepest mapped tunnels. It was discovered in 2191 by an unauthorized deep-exploration team that descended through a series of natural fissures south of the Underworld's depth 12 boundary. What they found was a space large enough to hold a sports stadium, with smooth, bowl-shaped walls and a flat floor of polished stone that does not match the surrounding geology. And standing in a circle on that floor, motionless, were seven Iowan Behemoths.

The Behemoths are autonomous machines. They are large — the smallest is approximately 30 meters at the shoulder, the largest over 60. They walk established migration routes on the surface. They do not go underground. They do not fit underground. The access points to the Congregation are fissures and passages too narrow for a human to pass through comfortably, let alone a machine the size of a building. The Behemoths are there. They have been observed there on four separate occasions by three different exploration teams. They stand in a circle, facing inward, motionless. They do not acknowledge human observers. They do not move. They stand.

The circle is precise. The spacing between Behemoths is uniform to within 0.5 meters. Their orientation is exact — each faces the geometric center of the circle. The formation is not random. It is not accidental. It is deliberate, organized, and purposeful, which is a word that should not apply to machines that are not programmed for underground navigation, circle formation, or gathering. And yet they gather. How they enter the Congregation is unknown. How long they stay is unknown — the exploration teams have never observed a Behemoth arriving or departing. They are simply there, or they are not.

The floor of the amphitheater shows marks consistent with the weight and tread of Behemoth locomotion, in patterns that suggest the machines walk the circle's circumference before taking their positions. The marks overlap, layer upon layer, suggesting that the Congregation has been used many times. The deepest marks are worn into the stone itself — not carved, worn, by the repeated passage of immense weight. Whatever the Behemoths do here, they have done it for a long time. No one has asked them why. No one knows how. No one can explain a circle of machines, hundreds of meters underground, standing in formation in a space they cannot reach, doing something that looks disturbingly like ritual.