I think there's a human head underneath my house
Have you seen it or something?
Someone told me about it.
If they are telling you the truth, then you should call the authorities. There could have been a crime committed that needs to be investigated.
I want to retrieve it myself and do an autopsy.
It's great you are very curious and want to further your learning with hands on experience, but this possible head is not legally obtained. It could have been from a murder and you could be tampering with evidence, so it's best to call authorities first or else you can be seen as an accomplice.