It was probably using mobile network base stations. These can sometimes span quite large physical areas underground.
Additionally I'd assume they're quite hard to automatically update: The crowdsourced updates these databases depend on only work when at least one device has both a GPS signal and can see a cell or Wi-Fi base station, which is obviously quite hard underground.
Additionally I'd assume they're quite hard to automatically update: The crowdsourced updates these databases depend on only work when at least one device has both a GPS signal and can see a cell or Wi-Fi base station, which is obviously quite hard underground.