Train rail tunnel. Original public domain image from Wikimedia Commons https://www.rawpixel.com/image/3285735/free-photo-image-background-tunnel-architectureFree Image from public domain licensePhoto shows a side view of the Astron device. A single-layer solenoid, 92 feet long, is built around an aluminum vacuum… https://www.rawpixel.com/image/3322782/free-photo-image-aec-astron-buildingFree Image from public domain licenseView of an exploratory tunnel dug by the 25-foot-diameter tunnel boring machine at yucca mountain. https://www.rawpixel.com/image/3322876/free-photo-image-banister-cc0-creative-commonsFree Image from public domain licenseCulvert at Canyon Creek. 185 feet long - 12 feet span. by Alfred A Hart https://www.rawpixel.com/image/14256159/culvert-canyon-creek-185-feet-long-feet-span-alfred-hartFree Image from public domain licensePhotograph of the Interior Hull of a Dirigible before Gas Cells were Installed, ca. 1933. Original public domain image from… https://www.rawpixel.com/image/8802793/photo-image-light-flare-vintageFree Image from public domain licenseU.S. Army Pfc. Nicholas Broad, assigned to the 336th Firefighter Detachment, navigates a confined space during Combat… https://www.rawpixel.com/image/3317939/free-photo-image-84th-training-command-86th-division-americas-army-reserveFree Image from public domain licenseA section through the roadway of Holborn Viaduct, London: looking east, showing the middle level sewer. Wood engraving after… https://www.rawpixel.com/image/14006671/image-cartoon-person-artFree Image from public domain licenseChambre du rue Saint-Denis by Nadar Gaspard Félix Tournachon https://www.rawpixel.com/image/14271388/chambre-rue-saint-denis-nadar-gaspard-felix-tournachonFree Image from public domain licenseView of Marseilles Tunnel Construction by Nadar Gaspard Félix Tournachon https://www.rawpixel.com/image/14272614/view-marseilles-tunnel-construction-nadar-gaspard-felix-tournachonFree Image from public domain licenseSection of the Thames embankment in 1867: the position of the low-level sewer is shown. Wood engraving. https://www.rawpixel.com/image/14013080/image-person-art-skyFree Image from public domain licenseÉgout sous la rue de Châteaudun by Nadar Gaspard Félix Tournachon https://www.rawpixel.com/image/14272015/egout-sous-rue-chateaudun-nadar-gaspard-felix-tournachonFree Image from public domain licenseChambre du Pont Notre-Dame by Nadar Gaspard Félix Tournachon https://www.rawpixel.com/image/14272054/chambre-pont-notre-dame-nadar-gaspard-felix-tournachonFree Image from public domain licenseÉgout sous la rue de Châteaudun by Nadar Gaspard Félix Tournachon https://www.rawpixel.com/image/14272035/egout-sous-rue-chateaudun-nadar-gaspard-felix-tournachonFree Image from public domain licenseGalerie d'accès aux égouts by Nadar Gaspard Félix Tournachon https://www.rawpixel.com/image/14272193/galerie-dacces-aux-egouts-nadar-gaspard-felix-tournachonFree Image from public domain license[Untitled photo, possibly related to: Rocks coming out of gold dredge after ore has been removed. Nye County, Nevada].… https://www.rawpixel.com/image/12270726/photo-image-gold-road-public-domainFree Image from public domain license