What is this cap that is causing me to leak coolant?

[utahsaint]

Hi folks, I am new here so forgive me if I'm posting this in the wrong forum.

I have a 2000 GTP that is leaking coolant from under the motor where this brass colored round cap used to be. The cap is about 1.5" in diameter.

There are two of them, one is still intact on the engine block. This should normally be under the car near the O2 sensor (I think?) below the rear spark plugs, but it was kind of laying down near the oil pan as it has fallen out somehow

I've never seen this part before. Can anyone tell me what this is? How the heck did it fall out? Any ideas on how to get this fixed? The car is not drivable like this, the coolant just pours out from here as soon as the car is started.

Any help would be really appreciated.

Thanks!

 

[TorqueAutoMotiv]

freeze plug need some black rtv and a new plug theres a tool too knock it in with but you dnt need it

 

[blueguy]

Holy crap...thats like saying I have an oil leak and the filter is cut in half...