Rats don't live in the ground, gophers and ground squirrels do.

Rats are likely in walls, an attic but I'm not familiar with them living underground either.

Our neighbors had some that were going unchecked until they sold the house. Our dog caught one or two and we started seeing them at our fence. We put out Rat Zappers -- a battery operated zapper and placed them so they would be least likely to get other critters. I think we got 7-8 rats. One morning I opened the door and smelled skunk. The zapper had been flung a number of feet and was upside down and facing the opposite direction. I actually laughed out loud but felt sorry for the poor fella. It wasn't enough to do more than make him angry -- no skunks were anywhere to be found. (for anyone feeling even sorrier for the skunk -- they get even. they sprayed our dogs four time in the summer after the rats left).

Because of the dogs as well as neighbor dogs and cats, the squirrels and such, we didn't want to do poison or traps. The zapper worked for us. I'm not sure what the exterminator used next door. The zappers are available on Amazon. Good luck. Call a pro if you need one.

Are you having a problem with rats causing you harm or are you wanting to kill them "just because"? That's not being critical because, as we all know, Norway rats and brown rats deserve extremely violent deaths.

You say rats and you also say pack rats. Very different things. Which did you find?

Both live outdoors but "domestic" rats tend to not so much and are not often found living in the ground. Conversely, pack rats tend to not live indoors but can. Neither of these tendencies are hard and fast rules.

Pack rats indoors are far more common in new construction (VERY new construction) because they are extremely territorial and will move into a house under construction because it's being built on their turf. It's more appropriate to think of them as large gophers than vermin habitation rats. At a pound apiece, they're bigger than some cats and bunnies, so expect collateral damage from traps. Pack rats are relocatable.

"Domestic" rats are just vermin. Kill them. Kill them with fire.

First of all, before you panic, identify the exact species.

Next time you find one dead, take a couple of good photos with some sort of size clue in them. You need one full body and tail, one close of the tail (hairy or bald?) and one of the head and ears for size and shape.

"Tree rat" or "black rat" ... Rattus rattus
Norwegian or "sewer rat ... Rattus norvecicus

VERSUS
the cotton rat
pack rat
other native species

http://www.roofrat.net/ has great information

Here is a link that might be useful: Roof rat control

Please don't use poison. Use electronic traps. Install the batteries and handle them using surgical gloves, place in a dark spot near a wall. Leave it a few days before adding bait. And if the ground is dry, put a welcome mat of wet paper towels in front of the entrance so their paws will be wet before hitting the electrodes. Use fruit, nuts, shiny bits as bait. More at this link.

Here is a link that might be useful: Tucson newspaper article

This is veering slightly away from your question, but I notice that no one is wondering whether they may be "roof rats." If so, one thing to do is to cut back any fruit trees so that they're not near a wall or roof. I know there's a guy in Phoenix who's an expert.

thanks for the answers, everyone. I happen to have one of those Havaheart live traps, so I can set it up to see what kind of rats they are.

Funny thing is, I don't have any ripe fruit in my backyard (or vegetables.) I'll keep u posted.