The approximate shape of SCP-4594, marked in green, within Greater London

Item #: SCP-4594
Object Class: Euclid
Special Containment Procedures: An appropriate number of Foundation agents must be embedded as sewerage maintenance personnel in affected London boroughs and are tasked with reporting novel instances of SCP-4594-1 and 4594-2 to Site-95. Non-Foundation personnel and civilians observing or otherwise interacting with SCP-4594 are to be amnesticised and released.
The location of the primary SCP-4594 mass must be tracked via ground-penetrating radar. A minimum of one weekly rodent-based exploration of SCP-4594's interior is to be conducted with the purpose of identifying any deviation from the established characteristics of the object.
Description: SCP-4594 is a tubular organic structure resembling a human esophagus located below the city of London, UK. SCP-4594 forms a loop 380 km in length that extends across the Greater London region. The majority of 4594's mass is located at an average depth of 25 m below the surface. However, shorter tubes extend upward from the primary structure at irregular intervals and terminate in either sphincters (SCP-4594-1), which expel matter from the object, or mouth-like cavities1 (SCP-4594-2), which accept matter. These structures are seamlessly connected to the city's sewerage infrastructure and are the only conventional means of access to SCP-4594.
SCP-4594 is motile. In addition to undergoing continuous peristalsis2, the object is capable of large-scale movement by using its own mass to abrade the surrounding earth. In this manner, SCP-4594 has adapted to the changing subterranean infrastructural landscape of the city by selectively avoiding certain features, including the London Underground, while remaining in proximity to the evolving sewerage network.
SCP-4594's interior is host to an indeterminate number of rodents and other small mammals that use the object as a means of travel. Movement within 4594 occurs by utilising the object's peristaltic contractions to achieve forward propulsion. The majority of the population consists of house mice and brown rats, but cats, dogs, foxes, moles, and Eastern grey squirrels have also been documented. Despite the high incidence of crowding, SCP-4594's occupants are docile and do not actively interact with one another, instead remaining limp while within 4594. There is no evidence to suggest that animals observed in SCP-4594 are anomalous in origin.
SCP-4594 has been known to the Foundation since 2013. However, an examination of administrative records as well as disparate personal accounts of city residents suggests 4594 may have existed in some capacity since the advent of London's modern sewerage network in the late 19th century.
Incident 4594-1: A rodent-based exploration of SCP-4594 on 15th March 2019 was interrupted after the animal in question (a house mouse, designated R1871) was swallowed by a prominently-eared humanoid entity emerging through the solid mass of 4594. R1871 was approaching the sewers below the Royal Albert Hall at the time. Although the video feed was compromised in the process, audio transmissions were still received at Site-95 for several hours following the initial incident. A partial transcription is provided below.

<Begin Log>
<14:13:06> Rushing air.

<14:13:14> Brief splatter. R1871 breathing but otherwise silent. Deep thumping, similar to a heartbeat, in the distance.
<14:13:41> Approaching footsteps which abruptly cease. Sniffing followed by prolonged, high-pitched giggling.
<14:13:57> Heavy grunting.
<14:14:01> Wet burst followed by dripping and a soft thud. Giggling continues.
<14:14:12> Footsteps interspersed with scurrying.
<14:15:03> Muffled orchestral music and singing. Thumping intensifies.
<14:15:05> R1871 begins squeaking.
<14:16:20> Singing and orchestral music become clearer. R1871 matches the rhythm with its vocalisations.
<14:17:31> Thumping becomes louder. R1871's squeaks indistinguishable from deep, joyful humming.
<14:52:46> Thumping reaches greatest volume. Music and singing drowned out.
<15:38:22> R1871 screams, before breaking into song.
<17:59:58> Loud, extended belch.
<End Log>

At the time of transmission, the Mountbatten Festival of Music was being held at the Royal Albert Hall. However, no suspicious activity at the Festival was reported. Excavation beneath the Hall is ongoing.

Footnotes
1. Although vestigial jaws and teeth often accompany observed cavities, mastication does not occur.
2. Peristalsis is the contraction and relaxation of muscles that propagates in a wave down a tube. In the process of digestion, peristalsis propels material along the gastrointestinal tract.

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