By Maggie Shiels
Technology reporter, BBC News, Silicon Valley
More than 20 hours of video are uploaded to YouTube every minute
YouTube is making the tens of millions of videos it hosts more accessible to the deaf and hard of hearing by putting automatic captions on them.
The Google-owned company said this use of speech recognition technology is probably the biggest experiment of its kind online.
Previously captions were only on a small amount of content.
"A core part of YouTube's DNA is access to content," said the firm's product manager Hunter Walk.
YouTube said by opening all this content to those who have not really been able to access it in the past should democratise information and "help foster greater collaboration and understanding".
Initially the feature will apply to English language videos, with other languages being added in the coming months.
In November last year, YouTube rolled out automatic captions to a handful of partners including the University of California, Berkeley, Yale University and National Geographic.
The technology behind speech recognition has been around for about 50 years, said Google engineer Mike Cohen, and has finally become good enough to be used on a large scale.
"I have been working on speech technology for 25 years," Mr Cohen told the BBC.
"There have been steady improvements and this is the culmination of lots of work over years and years. We have had to work on a wide variety of problems like accent variation, background noise, the variation in language, in pronunciation."
Engineers say this example shows the technology has a mind of its own
The project team stressed, however, that the product is not perfect.
In one demonstration, software engineer Ken Harrenstien illustrated how the technology mistook the words "sim card" for "salmon" when Google executive Vic Gundotra addressed a developers meeting.
"It is not a complete solution but it is a step on the way to the real solution," he said.
"It's difficult to get every word exactly right but sometimes that doesn't matter and other times it's amusing."
Mr Harrenstien has worked on the project for the last five years. As someone who has been deaf from when he was a child, he said the launch of this feature was a big deal personally.
"This is huge. It is what I have dreamt about for so many years. The fact that you can now go on to any video online and expect to see captions is unbelievable and the fact I had a part in this is great."
Reaction to the addition of these captions has been very positive.
Students from the California School for the Deaf, in Freemont, made a video to show how much of a difference this tool means to them.
Angel Harrington signs her thanks to the YouTube team
"We felt like we weren't part of the world. We felt excluded," said Angel Harrington.
"Now we really can completely understand what is going on and we feel like we are on an equal playing field."
Ben Hubbard from Berkeley said this tool is a great way to open up the more than 500 courses it offers online.
"We are always looking for a way to extend the reach of this stuff and extend the reach of the content to a whole new audience."