A U-shaped tunnel was dug under the National Library of Wales early in WW2 and used by the British Library and National Library of Wales for storage of important artefacts.
THe entrance was on Llanbadarn ROad, possibly around SN 592 814
Cambrian News What lies beneath… Julie McNicholls Vale was granted access to a tunnel used to store precious items during the war
Wales Online The secret tunnels below a Welsh library that kept Leonardo Da Vinci artworks and treasure hidden from the Nazis The National Library of Wales kept some of the most important items in the world safe while London was bombed.
GEOGRAPH: SN5981 : Tunnel entrance, National Library of Wales
National Treasures: Saving The Nation’s Art in World War II by Caroline Shenton ISBN 9781529387438