Q. I thought maybe you could help me out with an interesting question. Because of all the rain recently we have a huge sinkhole in the back yard. We have been living in this house for six years, and I believe the house is about 30 or so years old. The hole is about 8 to 10 feet around with cinderblock sides. I can't imagine what it is. I was hoping maybe with your history or knowledge you might be able to explain it? If not I understand. I may lose a few backyard toys and hopefully not the big tree nearby.

A. What appeared is an old drywell that has collapsed in your yard. The old top failed because the soil covering it got heavy with rainwater and the cap couldn’t take it anymore. A drywell is a big hole dug into the ground and about a foot or so inside the perimeter they stacked concrete blocks in a circle without mortar to about a foot and a half from the top of the hole. Then they placed a heavy cover over it and poured gravel down the sides on the outside of the block and next to the dirt sides of the hole. To this they attached the outflow pipe from the septic tank then they graded earth over top of the whole thing and it disappeared from sight. More modern drywells have an inspection pipe—usually three or four inches in diameter that sticks up out of the ground in the center of the drywell location but the old ones when buried disappeared forever except in the memory of the installer who in your case is long gone. It was the septic system’s dispersal system prior to your home being connected to public sewerage. They can be very deep—thirty feet or more in some cases.

They may have filled in or removed the old septic tank itself when the house was connected to sewer but the drywells tended to remain abandoned. Frequently, the hook-up to sewer took place after the original homeowners were gone and the new folks didn’t know they even had them and thought that their dispersal system consisted of gravel filled trenches called leech-fields that didn’t need any sort of filling when abandoned. Remember, all this took place prior to the days of the county keeping detailed records of residential septic system installations.

I personally think abandoned drywells are quite dangerous. Imagine finding that drywell the hard way-- by falling into it. I guarantee you unless someone sees you fall in your screams will go unheard and you will drown! Depending upon the size of the original system you may find that you have more than one drywell out in that yard. Contact a septic inspection outfit who can probe you r yard looking for any other surprises like the one you had this week lurking below the grass. If they are lucky they might locate the old 3 or 4-inch lines that connected the septic tank to the drywells and follow them to see where they lead.

Don’t go near the edge of that hole and until it’s filled—the sides may be unstable. Erect a barricade around it so no one can wander near it. You need to fill that hole in immediately to make it safe. I think the best way to do it is with sand, gravel or recycled concrete as it compacts immediately. Soil-- unless compacted on the way up in twelve to eighteen inch lifts-- will continue to settle for years and you'll be filling, filling, filling. Look in the phone book under Sand and Gravel and call around. It’s not going to be cheap. Fill in that hole ASAP-- it is a hazard and you don't need the legal liability. Don't delay.