SAN FRANCISCO, California - Hundreds of thousands of disturbing videos of young children have been posted online, not in some dark corner of the internet, but on YouTube. 

The website is finally cracking down. It was a bottom-line decision.

Under pressure from advertisers, YouTube -- owned by Google -- removed more than 150,000 videos with images of children apparently being assaulted.

"At the end of the day, large tech companies are run by the bottom line, and I think YouTube has been an incredibly successful platform for Google, and they have simply not paid enough attention to some of the bad stuff that happens on that platform," said James Steyer, CEO of Common Sense Media.

YouTube began paying attention as advertisers pulled their ads, complaining about child-endangering videos.

In a statement Tuesday, YouTube said it is "implementing age gates for videos that are clearly geared toward mature audiences and reducing the incentive to produce this content by making 2 million videos ineligible for ads."

"It's a new type of child abuse -- child sexual abuse exploitation," said Mary Pulido, executive director of the New York Society for the Prevention of Cruelty to Children. "Children can't unsee something. Once they look at an image, it is there in their brains forever."

The problem on YouTube is that with some 300 hours of video uploaded to the site every minute, identifying offensive material is a constant challenge. YouTube has largely depended on computers to determine what is appropriate.

"A machine, a computer, cannot do that as well as a human being," Steyer said.

"So do you have to hire hundreds and hundreds and hundreds of people to look at everything?" CBS News asked.

"If you have to hire hundreds and hundreds and hundreds of people to look at everything, they should, because Google is coining money," Steyer said.

While YouTube is taking steps to protect children, experts say the first line of defense is at home -- that parents have to be aware of what their children are watching and should limit the amount of time children spend watching online videos.