His mother, Amy Blyth was strolling with Theo on Sunday morning when the toddler disappeared through the drain cover, as it flipped over and then closed on top of him.
The 23-year-old yanked off the metal cover and leapt down the shaft to lift her son out of the deep hole.
Theo was left covered in sewage and calling out for his mother after the nightmare accident at about 9:0 in Ashford, Kent, near where Ms Blyth’s mum lives.
CCTV on a nearby house captured the bone-chilling moment the little boy disappeared from view and Ms Blyth sprang into action to grab him from the sewer water.
Describing the “horror film” moment, she said: “My first thought was ‘oh my God, he’s dead’.
“I yanked the drain lid out, and I just saw him down there, screaming my name. There was sewage up to his knees.
“I jumped down the drain, but it’s not big enough for me to bend over to grab him. I had to sort of arch my body to grab him and pull him up.
“I don’t know how I did it. My body went in ways I’ve never bent before, just to get him out of that drain.”
She stood on ledges at the side of the sewer – that appears to be at least 5ft deep – to reach down to her son who had fallen to the very bottom.
Ms Blyth added: “It was so deep you couldn’t see my head from the ground, and I wasn’t even on the floor.
“Just as I pulled him up I think someone flushed a chain because we were soaked – with I think, possibly, wee. Our clothes are absolutely disgusting.”
Amy emerged from the drain “screaming for help”, while Theo was in an apparent state of shock, and “wasn’t really making a noise”.
The terrified mum rushed the little boy – who is said to have swallowed some sewer water – to Ashford’s William Harvey Hospital, where medics diagnosed him with mild concussion, but x-rays revealed he miraculously got away without a broken bone.
Personal trainer, Ms Blyth, said: “It could have ended up so much worse. If he’d gone down head first, I don’t even know if he would be here.
“It makes me feel sick to think of it. It’s an absolute nightmare – I’m petrified of drains now. I still feel really shaken from it. I just can’t really get over it.
“He literally went straight to the bottom, and I thought I was going to lose him. It was actually like a horror film.”
She added: “We were just walking out for him to go to his dad’s house. I saw the drain was a little bit loose, but I didn’t realise what level it was loose.
“He ran to the drain, stood on it, and just fell the whole way through. And then the drain lid shut on top of him.”
Ms Blyth, who lives in nearby Tenterden, Kent, said she dreads to think what might have happened had a child fallen through the drain cover unseen.
She added: “If he’d been on his own, or if it had been another kid on their own, we wouldn’t have found him.
“We wouldn’t have known where they’d gone. You couldn’t even hear him because it’s sound-proof.
“He could have fallen down and I’d have gone running round the block trying to find him, not in a drain.”
Theo is now home and doing well, but Ms Blyth complained to Southern Water and said she “needs answers” as she believes the drain cover has been faulty for some time.
She added: “My little boy shouldn’t have fallen through a drain. It’s not acceptable. We need some answers to why it wasn’t fixed.”
It is understood Southern Water is working to establish who is responsible for the drain and manhole.
Around four investigators were seen in the area on Monday, looking into the cause of the accident.
The utility company confirmed it is investigating and taking the matter “very seriously”.
A spokesman for the firm said: “We are aware of a situation involving a child and an open manhole.
“This is clearly an upsetting situation for those involved and our team is on-site. A dedicated customer liaison officer has been assigned to support the family.
“We are taking this matter very seriously and our first priority was to make the area safe. Next steps are to understand exactly what happened.”
Southern Water has been contacted for an update into its investigation.