For anyone who's noticed, we have a very bumpy grass runway... since about October. I thought it coincided with the last time the field was cut and maybe Brian had aerated the grass. If you muck with a mound you'll see a tiny hole which, again, I thought had been done by something mechanical.
I believe these mounds are worm castings. This means we have a very healthy grass runway... well... healthy for anything except airplanes... especially with small wheels.
I've done some research and it's a very common occurrence, although I've never seen it before at TEMAC. The image below is not from our field but just a random photo found on the web... and it matches what we have exactly. If it happens again next year maybe a fall roll will help.
Definitely a sign of Lumbricus terrestris Stouffvillensis ;D
Andy