what a load of old galls

The ground down my local woods is covered in fallen oak leaves right now, and some of them are plastered in these mysterious "blisters".

What they are actually are wasp "galls".  A wasp has laid its eggs and these galls then provide shelter and eventually, after falling to the ground, food for the developing wasp.  You can see like little pin-prick marks where some have already become detached from the leaf.

Nature is truly fascinating!

Nikon Z6, Micro-Nikkor 55mm f2.8 AiS with PK13 extension tube
5/8th of a second at f11, ISO 100
tripod, self-timer release
11 image focus bracket stacked in Helicon Focus