This Civil Engineering special issue is on underground construction. It explores some of the latest developments and innovations that are transforming underground construction across the world. The wide-ranging papers cover various ways in which underground spaces can meet society's future needs, and the innovations in underground construction technology that are helping to improve safety, delivery and environmental performance. The issue has been supported by the British Tunnelling Society (BTS), an Institution of Civil Engineers associated society, which celebrates its 50th anniversary in 2021. Since its first meeting in March 1971, it has since grown to become one of the world's most vibrant gatherings of professional tunnellers in the world, providing industry guidelines, codes of practice, training, conferences and advice to government on all aspects of underground construction.