I found these pillars of dirt and pebble in one of my planters!  This planter has some thriving squash plants.  They have a massive nest in a planter that is about 4 feet tall, just 3 feet away from this planter.  They are in the smaller squash planter often as well, but they spend most of their time in the larger one, it seems.  The larger one has corn and snow peas growing in it, which seem to be thriving along with the ants; it has dirt piled up against the side of the wooden barrier, presuming to make it easier for the ants to to climb out?

Unfortunately, these pictures were taken yesterday evening, the ants had already gone underground for the night.  I was hoping to get more photos with ants present, but it rained today, these structures may be gone now.

I suppose it is also possible that my roommates or a sneaky neighbor snuck in and built these things.  They are only 2 to 4 cm’s tall, approximately.