These aren’t ant hills at all. You’ve had a swarm of night crawlers nest and hibernate in your lawn over winter. Night crawlers are worms. However they aren’t your garden variety worm. These ones are much larger. They’re the thick long worms that fishermen use. In the fall they burrow vertical holes in your lawn where they hibernate until spring. They usually emerge almost all at once, overnight of course. They don’t usually cause any serious or permanent damage although they can sometimes leave your lawn very bumpy, like you’ve got golfballs under your lawn when you walk on it. Given this years’ extremely wet conditions, the soil has remained very pliable. It’s not likely you will find this bumpy result of their activity. When the bumps in the lawn do occur, the best remedy is an aeration to reduce the compaction in the soil. In extremely serious or dry conditions a second aeration may sometimes be required in the fall.