Virgin Oceanic’s Voyage to the Bottom of the Sea - IEEE Spectrum: The pilot’s head will be within the pressure-�resistant transparent dome on the front of the pressure hull. This will give him a much better view than researchers get in government subs, which sport just a couple of small portholes. The original plan was to use high-strength glass for this component, but when the glass dome was tested in a pressure chamber, it cracked under the pressure it would experience at just 5000 meters. The thick glass is now fissured all around the edges where it meets a titanium ring.
In the trench, such cracks wouldn’t be just a disappointment, they’d be deadly. So Virgin Oceanic hired a company to make a new dome out of a 1270-kg ingot of synthetic quartz. Then a company that makes lenses for NASA telescopes spent one year hollowing it out into a hemispherical shape. This new dome will be tested in a pressure chamber at Pennsylvania State University this spring.