When I first experienced this I was below a beaver dam lowering pond level to our disguised outflow tubes. Had never seen the form posted in winter surprise...will be on the lookout from now on, but hopefully with more than cellphone available. The form from center rotting hung tree falls(of the sort Thierry has posted) looks so exquisite...hope to experience that form one day! Now that the hydraulics of the process have been illustrated I'll be wanting to try to find some of those tubular conduits from ground to egress more often when the conditions give promise...Wow...grateful to Thierry!