It seems rather likely that a strong burrowing animal could make it through just fine. Next question is likely whether that animal could clear a path for others. That's more problematic, the mud is slick and loose enough that any tunnels are likely to collapse right away.
You could in theory work to shift the mass of the blockage down the tunnel bit by bit. But that will take a great deal of time.
Edited: daishain on 26th Jun, 2018 - 12:36am
Dungeon Master: Did anyone make it out of the tunnel before the mud appeared? I had stated that Morgaine was going that way, thinking she was last of members to enter the tunnel and also know others were headed that way as well.
Since I declared Poljen would play Tail-End Charlie, he did NOT use his Bonus Dash as they left the cavern for the tunnel. And since he neither attacked nor took damage, his Rage has dropped. When Neronvain makes his announcement to Perry, can any of the rest of us hear that, or are we too far away?