"Darwin spent over 40 years studying earthworms, and wrote a book about them and their earth-moving abilities (The formation of vegetable mould through the action of worms).
He found that in one acre the earthworms were bringing up 10 tons of worm casts a year, and an earthworm can digest its own weight in soil every 24 hours. In tropical soils it is thought that the amount is at least 20 times more as the worms are larger and are more active. This means that as much as 50% of the soil passes through the gut of earthworms each year, greatly accelerating the internalisation process of organic particles.”
I also read often that per surface acre earthworms beneath the soil outweigh the animals above (including cattle). I can’t find the source for it though, so, as the ancient Romans, take it with a grain of salt (but please, not near the earthworms).