Sinkhole mystery

There seems to be a mysterious depression developing in the corner of the backyard. I was digging a hole to plant my tree and at the bottom of my foot deep hole there was another hole, a dark black one that the dirt fell into.  This is the stuff of nightmares( at least for me anyway)! I stuck my shovel into it and it kept going, so I quickly pulled it out and stepped away from the hole.  That's when the hairy knarled hand reached out and grabbed my ankle and dragged me into the black smelly hole....Sike! That part was fiction!  Anyway..My heart pounding, I shoveled the dirt back into the hole and called my husband at work.  Yikes!! When you stand back away from the depression it's noticeable enough to be alarming. So I called the Maryland Geological Survey and left a message for them to call me back...which they haven't yet.  I'll call again today to see if they answer or if my message fell into a sinkhole like the one in the backyard.