I own a family house that used to until about 20 years ago have a sewage collector pit in use. It has been abandoned for about 20 years now. It has concrete walls and roofing and gravel on the bottom. It was never disinfected or treated in any way.

My question is whether it is safe to go down inside the pit or it could be infectious?

EDIT: My intention is to use it to dispose of plant matter.

EDIT2: 

1) I originally asked this question *after* I went in because I was afraid it might be infectious. Gases accumulating never occured to me, which was stupid of me, but now I know, since many responses and comments pointed it out. This is something I will take into account and make sure everyone who might ever want to do anything with the pit does too. The pit itself is structurally sound, there is no chance anyone in the next 50 years will ever fall into it by accident or anything like that.

2) The plant matter too I already put in when I asked this, since, as I've written above, it has not occured to me this could pose a danger. So it was a done deed, not an actual decision I took _after_ being aware of the possible issues. A very important additional info is that this 'plant matter' is actually half rotten (as in brown rot) dry wood, it is not something that will vigorously decompose and produce a lot of gases very quickly. Another important aspect is that this pit is safely closed (though it has a grill covered opening with a diameter of about 15 cms on its roof that exits into the garden so it is very definitely not a hermetically closed space) and there is literally no chance anyone ever going inside without taking precautions. I know because it's either me, or someone I might sell the house to, but if I sell it I will most definitely tell them about the pit and the possible accumulation of CO2 and the precautions they need to take.