Filament suitable for direct burial I'm working on a project with will be buried in soil. It's an enclosure for a sensor that will be potted inside the 3D printed part. What filament will give me the longest life in soil? 

ETA: burial will be permanent, and I'd like it to last at least five years.

ETA: The printed part will provide mechanical support for the sensor, so it needs to retain most of its mechanical properties.
I would recommend PETG - only because it is structurally similar to the plastic used in the bottles that last forever, and most PETG is food grade - implying that its chemical stability should be reasonably good...