Without being there, we can’t tell you for sure. But there’s plenty of possibilities—most of them indicative of something that was formerly at that location but now gone. Here’s eight we know of: 

1) Electrical conduit - If there was once an above-ground pool or shed in the backyard, there may have been electric service through a conduit pipe coming up out the ground to a receptacle outlet box for it that is also now gone. The pipe would have a diameter of 1” or less, and either metal or gray plastic. Do you see any snipped wires down in the pipe?

2) Underground fuel oil tank - If the home is pre-1970, and there is a pair of galvanized steel pipes within a couple of feet from each other as in the photo above, it may be evidence that a storage tank for a long-gone fuel oil furnace is still in place underground. There will be a mushroom shape over one pipe (vent) and a hinged flap over the other (fill) if the top pieces are still intact, or it may just be two rusty pipes with threaded ends if they have been removed. 

3) Sprinkler system - Small diameter plastic pipes sticking up at multiple locations around the yard are probably the remains of a long-gone lawn sprinkler system. 

4) Abandoned well - If the pipe is a larger diameter of two to six inches with a threaded end and goes way down into the ground, it may be an abandoned well. Do you see the reflection of water at the bottom when you shine a flashlight down it? If so, it should be sealed to avoid contamination of the aquifer below. 

5) Location of former LP-gas tank - A curved copper pipe with a threaded female fitting at the end may be for a former above-ground LP-gas tank at that location.

6) Underground water shut-off valve - When there’s a water pipe underground with an in-line shut-off valve, a large diameter plastic, often light green, will be installed over it so you can reach down in the ground to use the valve. However, over time, leaves and dirt tend to fall into the pipe and you may have to dig down a little ways to see if there is a valve down there.

7) Clean-out for the main drain line to septic tank or sewer - This pipe will be larger-diameter plastic and should have a removable cap on it. But if the top is missing or you remove it, you will be looking down a few feet to a horizontal pipe that will have a small stream of fluid flowing at the bottom when any plumbing fixtures in the home are in use.

8) Just a piece of pipe - If you give the pipe a tug, you may be surprised to find that it’s just a short piece of pipe that someone stuck in ground.

    Also, see our blog posts What are the abandoned pipes sticking out of the wall in my house? and What are the pipes on my roof?

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house? 

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?  

• What are the code requirements for plumbing vent terminations?

• What is the difference between water service pipe and water supply pipe?  

• How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?

• What is a P-trap?

• Why is old galvanized steel water pipe a problem for homebuyers?

• What does polybutylene pipe look like? Why is it a problem? 

• Which water pipes are an insurance problem and possibly uninsurable?

• Can you connect CPVC pipe directly to a gas water heater?  

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.