Second starts on 10th January:
https://yesterday.uktv.co.uk/shows/underground-worlds/

Previous series is on demand:
https://uktvplay.uktv.co.uk/shows/underground-worlds/watch-online

This may also be of interest:
https://uktvplay.uktv.co.uk/shows/secrets-of-the-london-underground/watch-online/

Series 1 episode 1 started with Gibraltar (tunnels & St Michael's cave), Slovenia (Postojna - show cave, salamander & divers), plus London & Liverpool sewers

Cwmorthin was on tonight's episode.
https://uktvplay.uktv.co.uk/shows/underground-worlds/watch-online/6292178801001