I think there is a cat underneath my house

I believe their is a cat underneath my house. I've seen this cat several times over the course of a week near my house, going underneath the foundation because I have a gap missing right now. The cat that I have inside my home keeps staring at the vents in my bedroom and its creeping me out. How do I go about trying to get the cat out? I'm concerned for that cat just as well and thinking that it might have babies under there.

I think I would put food in your front/back yard, that is, away from underneath the house. Do that on a regular basis. (If she is nursing, she needs the calories.) Then I would try to trap the cat and take it to a low cost spay/neuter clinic, to get their opinion as to whether the cat is nursing. Unless of course you're able to determine that yourself by examining her/his belly.
Although by the time you're able to trap the cat, she may be bringing the kittens to your 'diner'.
Unless the mother moves the kittens (if any) into a more accesible place, there isn't much you can do about getting the kittens in a safe place.
When I was TNRing (trap-neuter/spay-return) on a regular basis, I can't tell you how many times I swore I trapped a female cat and turned out to be male. (Though males of course should be neutered.)
Oh, and your cat may be hearing a mouse or similar critter, not kittens.

I agree that I would begin by feeding the cat.  If possible, take something nice and tasty out (like lunch meat, tuna, salmon, sardines, Vienna sausage, or canned cat food), and stay outside while she eats.  You may need to be pretty far away, and sitting down so you are on a lower level could be helpful.  If you can do this every day, you will find you can move closer and closer to her until you are finally sitting pretty near her while she eats.  You might talk to her very softly so she can get used to hearing the sound of your voice.  

I know that most people advise to never touch an animal while it is eating, but it has been my experience that that is actually the very best time to first touch them, as long as you are observant and use common sense.  They are hungry, the food is good, you've been around them every day while they eat and you haven't done anything to hurt them . . .  I believe that every single feral cat I have tamed was first touched while it was eating.  Take your time, watch her to see what she is ready for, and you may find that pretty soon she will accept your touch without having to be eating.  

I think that feral cats can make wonderful pets.  They may never be comfortable in the house, but if you have a fairly safe yard they can be quite happy there.  Once you have gotten her to the point where you can touch and pet her and she trusts you, you can take her in and have her spayed and vaccinated for rabies.  You may not be able to pick her up, but if you take a crate out and put her food in the back of it, she should go right in.  If you want you can feed her in the crate for a period of days or weeks so she is used to it.  When you are ready you can close the door and take her on in.  

Talk to the vet ahead of time, and see if you can do a walk-in appointment when you have her in a crate.  Many vets will be agreeable to this.  We used to have a vet that had a standing arrangement with us.  If we had a feral cat, we could bring it in any time we caught it.  We just had to call first and let them know we were coming.  He was also kind enough to spay and neuter them for us at the same fee he charged the Humane Society, which was discounted from his regular fee.  I believe that many Humane Society clinics will also take ferals without an appointment.  You would have to check with yours to see what their policy is.  Our local one says you can bring in a feral any time you have one.  You just have to show up first thing in the morning on the spay/neuter days.  

Just be aware that when you bring her home you'll have to keep her quiet and as inactive as possible for a few days.  This may not be to her liking, but it will be a great opportunity to spend a lot of time with her and help her to get even more used to you.  You may not be able to put her back outside until he incision has healed.  If she has dissolving stitches, you won't have to take her back to get them out.  You never know, she may find she likes being in the house.  :-)  

Please let us know how things go.  You have the potential of making one of the best four-legged friends you will ever have.  :-)  

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I just realized that you may not want to tame this cat.  I read your post again, and I think you just wanted to get the cat out from under your house.  I'm leaving my post as I wrote it, in case that is what you want to do.  But, if you just want the cat out, I would still start by putting something tasty out, and staying outside.  You might have to start out feeding the cat close to the house and gradually move the food farther away.  When you know the cat is out, because she is eating, you can block the entrance to under your house.  There you go.  Now you can choose whether or not to keep feeding her and making friends with her.  

Before you block the entrance to under the house, see if you can determine whether or not she is nursing.  If you can get a good look at her belly somehow that should let you know.  If she does not show any signs of nursing then you can block the entrance without worrying about trapping kittens under there.  If she appears to be nursing, I would leave it open.  She will eventually bring the kittens out.  It shouldn't take you too long to know how many there are.  You can block the entrance at some point when you know they are all out.  

Wow,thank you for being so kind.The under house cat may indeed be nursing,but sometimes its hard to tell.Is this a feral or stray?Could it be a neighbors cat?Is the cat keeping the field mice population under control?Unfortunately to distinguish this will take time.Patience is your best friend right now.As the other post stated don't block kitty's ability to get in and out.Keep us posted.