I don't get it

Two North Dakota good ol’ boys are out hunting, and as they are walking along they came upon a huge hole in the ground.  They approach it and are amazed at the size of it.

The first hunter says, "Wow, that's some hole, I can't even see the bottom. I wonder how deep it is?"
The second hunter says," I don't know. Let's throw somethin' down there, listen and see how long it takes to hit bottom."
The first hunter says, "Hey, there's an old automobile transmission over there. Give me a hand, we'll throw it in and see."

So they pick it up and carry it over and count one, two, three and heave it in the hole.

They are standing there listening, looking over the edge, when they hear a rustling behind them. As they turn around, they see a goat come crashing through the underbrush, run up to the hole and, without hesitation, jump in headfirst.
While they are standing there staring at each other in amazement, peering into the hole, trying to figure out what that was all about, an old farmer saunters up.

"Say there," says the farmer, "You fellers didn't happen to see my goat around here anywhere, did you ?"
The first hunter says, "Funny you should ask, but we were just standing here a minute ago and a goat came running out of the bushes doin' bout a hunnert miles an hour and jumped . . . headfirst into this here hole!!!!"
The old farmer said, "Naw, that's impossible . . .    I had him chained to a transmission."