Yeah they explained that a while ago (and I'm not criticizing you I'm agreeing) and Des was going to sacrifice his life by destroying "the Hatch" which is also The Swan Station.

And they pushed the button every 108 minutes, not every 90 seconds.
And did anyone else notice that all the "special numbers" they had to input added up to 108? Somehow I don't think these numbers are the last we'll see of them.