I went down in my crawl space and found it filled with sewage. I noticed that my Kitchen Sink has an overflow pipe that drains into the crawl space. My main pipe was clogged so all the water went into the crawl space. Is this normal? I was wondering maybe the kitchen sink has one of those gas pipes to allow the fumes to escape.

Any advice?

This is crazy, sorry. Nothing here is normal yet, not even close. The reason the clogged pipe is there is to drain the waste form the sink etc.

I was wondering maybe the kitchen sink has one of those gas pipes to allow the fumes to escape.

The "pipe" to vent the smell should be off that kitchen pipe and that drainage line. ( a closed system)

That vent needs to go to the roof if at all possible to vent that line so NO sewer gas gets in the house.

The first thing to do here is unclog that main pipe.

Look at the roof and can you see a pipe sticking out of the roof.

Of the problems I see there, and the knowledge you are lacking ( no offense) a plumber may be a good call here, unless you have the skills to fix and run drainage and or venting.

So its your call and again snake out the main line to clear.

The other question how/why was the sink pipe disconnected from the main drainage line, short of breaking that line going cheap quick fix.

This is crazy, sorry. Nothing here is normal yet, not even close. The reason the clogged pipe is there is to drain the waste form the sink ect.

I was wondering maybe the kitchen sink has one of those gas pipes to allow the fumes to escape.

The "pipe" to vent the smell should be off of that kitchen pipe and that drainage line. ( a closed system)

That vent needs to go to the roof if at all possible to vent that line so NO sewer gas gets in the house.

The first thing to do here is unclog that main pipe.

Look at the roof and can you see a pipe sticking out of the roof.

Of the problems I see there, and the knowledge you are lacking ( no offense) a plumber may be a good call here, unless you have the skills to fix and run drainage and or venting.

I snaked and unclogged the main line. Everything is working now. The question is, can i close this pipe?

I have Two sinks, with two traps under the sink. Both of these sinks connect to the main drain under the sink. However, there is a third pipe leading to this unclosed drain. I do have a vent pipe on my roof. Is this vent pipe off the main drain or could it be this third connection under my sink, which also might have been used along time ago as a main drain?

The other question how/why was the sink pipe disconnected from the main drainage line, short of breaking that line going cheap quick fix.[/QUOTE]
This third pipe did not disconnect from the main line since there is nowhere that this can connect to the line. I thought it might have been capped, but did not find any caps that fell off.

Looks like somebody left clean out plug open under your house. You have clogged drain and you need to snake it. Also, reinstall clean plug so you close the drain under the house. Spread lime over the wet area to help it dry out faster and prevent long term smell.