This war has seen trench warfare not seen since WW1. During WW1 both sides dug tunnels under enemy trenches, filled them with explosives and detonated them just before a major offensive.
There has been no suggestion or rumour that Ukraine or Russia are doing that. Which is odd. Particularly when places like Bakhmut have existing networks of mines, mining equipment and ex-miners.
A long winter of bad weather and stalemate on the battlefield would have been the perfect time to construct something.