Old New-Gate Prison & Copper Mine
A colonial copper mine later used as Connecticut's first state prison, with underground workings and surviving prison ruins.
DocumentedTicketedFee or pass1 to 2 hoursChecked July 2026
Category · 6 field files · 6 states
Underground sites
Places below the surface that are open to the public through an authorized operator, a park service, or a museum: buried streets, historic mines, cave systems, and prisons dug into rock.
Material: soil strata and a seam of copper. Brand imagery, not a photograph of any listed place.
- Access considerations
- Every underground file in the Atlas is reached through a ticketed tour, a timed entry, or a ranger-led route. There is no free-roaming underground access here, and the Atlas never publishes unofficial entrances.
- Safety considerations
- Cold, darkness, stairs, low ceilings, and uneven rock are the standard conditions. Carry light where the operator allows it and follow staff directions.
- Evidence expectations
- Underground files are documented sites. Where a tour also trades in legends, the file is labeled History + folklore.
- Source-check status
- Most recent practical-details check in this category: July 2026. Conditions change. Check the primary source before leaving home.
State coverage
Underground sites in Volume 01
Strataca
A museum 650 feet below ground inside a working salt deposit, reached by a controlled mine descent.
DocumentedTicketedTicketed2 to 3 hoursChecked July 2026Mammoth Cave
The world's longest known cave system, with human history, enormous passages, and routes ranging from accessible to demanding.
DocumentedGuided onlyTicketed2 hours to full dayChecked July 2026Soudan Underground Mine
Minnesota's oldest iron mine, interpreted through seasonal underground tours and surface mining history.
DocumentedSeasonalFee or pass2 to 3 hoursChecked July 2026Pendleton Underground
A guided route through spaces beneath downtown Pendleton, presented by a nonprofit tour organization founded in 1989.
History + folkloreReservation requiredTicketedAbout 90 minutesChecked July 2026Seattle Underground
Buried storefronts and sidewalks left below Pioneer Square after Seattle rebuilt above its old street level following the Great Fire of 1889.
DocumentedGuided onlyTicketed75 minutesChecked July 2026