At our farm we have a very old lawn around the house, and I appreciate every green inch of it, even though I know it is not very politically correct in current garden thinking. Oh well, did I say that before moving to Central Oregon I lived in Carson City, Nevada for 15 years? We lived in a ranch foreman's cottage on an old ranch that we managed, and there was no lawn to speak of, just gritty, decomposed granite with a teeny % of organic matter in it, enough to support all the "brushes": rabbit, sage & salt that is! We did have over 70 acres of flood irrigated pasture to take care of though.
No, I am happy to let the night crawlers cavort under my lawn. After all, they do make all of that turf more of a living thing, and maybe it helps to assuage my guilt at dumping all of the water on it to keep it green every summer.
So, back to the bumps - what to do?
First, rent a gas powered de-thatcher. Go two directions to really get a good thatch removal. Then, get out the mower and mow up all of the dried "strawy" thatch. (Best to use a bag mower if you can, but not absolutely necessary.) Lastly, rent a power aerator to core aerate your lawn. Leave the core "droppings" where they are - please resist the urge to rake them up as they are really good for the lawn, as it is just replenishing the organic matter to the crown of the plant. You are almost done, but if you really want to achieve "flatness" go back to the rental store and rent a lawn roller. After lightly watering your lawn, roll over it a few times to flatten out the last offending bumps. Fertilize and water, and voila, no more bumps!
After all this work, maybe you should see about hosting a lawn party. Croquet anyone??
Nah, I'll take daffodils and night crawlers if you don't mind.
