shaft sinking
excavating a vertical or near-vertical tunnel from the top down, where there is initially no access to the bottom
en-US/ˈʃæft ˈsɪŋkɪŋ/
en-GB/ˈʃɑːft ˈsɪŋkɪŋ/
excavating a vertical or near-vertical tunnel from the top down, where there is initially no access to the bottom