Backyard, as one word, is an adjective; it needs a noun to describe, and it doesn’t work on its own. (Example: The backyard pool is in the back yard.) Change it here to back yard.
Get rid of the ellipsis at the end and just use a period.
You are digging in your back yard to make a small garden when you uncover a hatch lid. You open it and shine your flashlight down.