Waze Beacons [1] do this for car tunnels. Some underground systems, e.g. in Barcelona IIRC, also use BLE beacons to help phones position themselves. [1] https:

Waze Beacons [1] do this for car tunnels. Some underground systems, e.g. in Barcelona IIRC, also use BLE beacons to help phones position themselves. [1] https://support.google.com/waze/partners/answer/9416071 https://support.google.com/waze/partners/answer/9416071