how to protect the underside

Can you install a proximity sensor under a vehicle? And what about small dogs or cats or other animals setting it off?

That may be worth a shot - just make sure the box is weatherproof.  What is under the vehicle that you need to protect?

Well I saw a deal on the news that people are carrying a battery powered saws all and in a few minutes they have an as scrap value $100 plus catalytic converter for one, and for two there was also a deal on the news showing  someone jabbing a screw driver in a gas tank to steal gas. So what do you think is under there worth protecting?  Cost to replace that converter would probably run into 200 to 300, and about 300 or more for a gas tank with a hole in it.  So that is close to 600 dollars worth of stuff to protect and that is only 2 items, if you have 2 gas tanks like on a truck, then add another 300 so that is close to 1000 now.

I would like to think a shock sensor would be enough protection against both of those.

The thing about cats and gas is that all cars have them.  If a thief walks up to a car with a flashing red LED from an aftermarket alarm then he will most likely move to an easier target.

Lucky for me my car sits low enough that I don't have to worry about anyone climbing underneath it any time soon!