Working with the colliery’s mechanical staff, Gill designed an oblong steel cylinder—affectionately called the Gill Capsule or Bathyscaphe . Dimensions were critical: 2 feet 2 inches in diameter and 3 feet 9 inches in height. It looked like a small diving bell. It had a hinged lid, a small perspex window, a single lever for the trapped man to operate, and a valve for air circulation.