Woman Trapped in Underground Sewer System

by Jason
(Orange, California, USA)

Around 1970 (I was five) I saw a horror movie or TV Show that scared litteraly the poop out of me.

A woman is trapped in an underground sewer system screaming for help. Two boys from a park eventually see her through a grill in the ground.

In the end, the woman is rescued. She's covered with mud and sludge, soon to be cleaned off with a garden hose in a driveway.

Can anybody help with name of this movie?

Thank You,
Jason