Isn’t it already happening? Is that what is creating the huge methane explosions and craters in Siberia?

Yes. Check the Yamal Peninsula in Google Maps and you realize this kind of cratering is a process that has been going on for thousands of years (since the ice age really!).

The scary part is that it's accelerating in on a timescale that's relevant to our needs as a species.

I think the largest deposits would take thousands of years to be affected, but there are shallow deposits that could be released, especially those near glaciers, where the glacier melting allows the nearby seabed to rise.

https://en.m.wikipedia.org/wiki/Clathrate_gun_hypothesis