Then about 2 years ago the depression started getting deeper. I assumed I had a broken sewer pipe and that it was leaching soil every time the water ran. When the depression had reach about 6 or so inches deep I poked at it with a shovel. The whole shovel head went through the lawn! After widening, I had a hole about 4 feet square and too deep to reach with the shovel handle. I couldn't believe I was standing on that with just the lawn supporting my weight.
Of course it turned out to be an old abandoned septic system. When the sewer system was installed on my street they just covered the septic tank with redwood boards (about 3 inches thick) and ran the pipe around the tank and out to the street. After 50 years or so the boards all rotted away.
The neighbors stopped by over the following week as I filled the hole with gravel. All of them noted that they too had curious depressions in their front yards.
randystone