Talk about a "holy s***" moment ... check out this insane video of a boy in China getting literally blown off his feet after a sewer explodes underneath him.

The wild footage was shot last Friday by a witness in China's Gansu Province, where you see the youngster walk up to a sewer opening and drop a firecracker inside.

Suddenly, the street detonates beneath his feet, sending him flying high into the air with all the debris. Soon, the kid comes crashing back down to earth on cement steps, causing him to suffer serious injuries.

Bystanders scurry up to him to see if he's OK as he lay still on the ground.

The boy was reportedly rushed to a hospital, where he was being treated for his injuries. The good news is he's expected to survive.

What's the lesson here ... don't play with firecrackers near sewers!