Answer: It sounds as though some of the roots of the tree were left in the ground when you had the tree removed. The mushrooms you're seeing on top of the soil are evidence of decomposing organic matter (probably tree roots). The soil is settling as the roots decompose and will continue to settle until most of the root mass has rotten away to nothing. You can remedy the situation by digging out the remaining tree roots and then filling in the hole and tamping the soil down. This fix is likely to be quite labor intensive. Or, you can continue to fill the area as it sinks. If you're trying to grow lawn in the area, refilling and reseeding might go on for quite some time if you leave the tree roots in the ground. Aside from turning the area into a raised planter (which will slowly sink but won't be quite so noticeable), there's not much else for me to recommend. As for the mushrooms - expect that they will continue to emerge and either knock them down and pick them up as they do, or simply mow them up each time you mow the lawn. Wish I had an easier solution for you!
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