Soil temperature

We need an intelligent ball we can bury in the ground. It monitors soil temperature over several weeks.
When we dig it up we can read off the time-stamped temperatures and graph them, An advanced model would send a radio signal above the ground for a receiver to capture. It could even be two way so that we could wake up the buried module to give us a burst of data.
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Unlike this sensor, we need to take our power supply underground.