New Basecamp: ajax or some super new technology?

We use Basecamp for project management and their new interface is REALLY fast.

It was interesting to me: looking at its speed and based on some hints from the interface, I came to the conclusion that it should be based on Ajax. Then I checked the site with Chrome and my suspicion was confirmed: indeed, the Network bar shows that requests are being added to page requests, and not completely reset as in standard page navigation.

But.

Ajax-based sites stand out most clearly for the fact that they have funky URLs: http://www.example.com/#page1 http://www.example.com/#page2 et al.

However, in Basecamp the navigation seems to be standard. Like this: https://basecamp.com/accountid/project1 https://basecamp.com/accountid/project2

So what is it? Is the new Basecamp ajax-based or is it some new hybrid that I am not aware of? Is this a web server setting? Is this a programming language (possibly Ruby)?

Hope someone else is as excited as I am. Thank.