THRESHOLD COMPLEX EXPLORATION LOG
Designation / Log ID: 24-U21
Apparent Date/Time: 27 August 2026 / 08:52 (local) — secondary stamp reads 14:17 09 November 1971
Entry Method: Double steel hatch labeled “F-11 FILTER HOUSE”. Hatch opens outward. Interior ladder rungs continue 1.4 m above the sill.
Environmental Baseline
Lighting: banked sodium-vapor lamps, color temperature measured at 2100 K. Intensity constant at 92 lux.
Ambient temperature: 22.7 °C. Humidity: 71 %.
Background sound: continuous pump drone at 61 Hz. Amplitude 31 dB.
Air movement: lateral flow of 0.7 m/s from north wall vents. Barometric pressure 1009.8 hPa on entry; 1013.4 hPa after 9 m of travel.
Spatial Observations
Chamber estimated 14 m long by 7.5 m wide by 4.1 m high. Laser measurement returns 21 m long by 5.2 m wide by 3.6 m high.
Floor: grated steel over open sump. Grate temperature 4.8 °C above ambient.
Walls: tiled ceramic. Original glaze intact on 60 % of surface. Tile temperature matches ambient.
Ceiling: exposed pipe runs. Pipe diameter inventory returns 18 discrete lines. Visual count remains fixed at 18 after three passes.
Repeating motif: every 2.3 m a pressure gauge is mounted at 1.6 m height. Gauge faces measure 0.18 m diameter. Needle positions are identical across all units.
Anomalies
Visual inventory of filter vessels returns 6. Physical inventory returns 6. Vessel height measures 2.4 m while ceiling clearance measures 3.6 m.
Floor remains level under spirit level. Measured water surface in the sump slopes 2.7° toward the south wall. Slope reverses after 5 minutes of continuous observation without change in physical orientation.
Flow rate through the primary intake measures 0.0 m³/min while valve position is fully open. Valve handwheel temperature measures 31.2 °C regardless of ambient.
Sound of pump engagement arrives 1.8 seconds before corresponding vibration is detected at the floor. Delay increases by 0.3 s for every additional 4 m of distance from the north wall.
Humidity at mid-chamber remains 71 % while condensation forms on the gauge faces after 4 minutes of observation. Condensation evaporates without measurable change in local humidity.
Entity Notes
None observed.
At the 11 m mark a localized humidity spike of 12 % occurs across a 1.8 m radius. Spike persists for 9 seconds then collapses. No visible source or airflow change accompanies the event.
Exit / Continuity Status
Exit achieved by retracing path to the double steel hatch. Hatch released with moderate resistance.
Exterior time on exit: 09:29 local. Elapsed interior time: 37 minutes. Wristwatch confirms same interval.
Residual effects: residual pump drone audible for 11 minutes after exit. Temporary inability to resolve gauge needle positions beyond 1.5 m lasting 7 minutes. No other reported sequelae.