Just looked it up. It's yet another climate tipping point. The idea is that warming ocean waters will cause deep frozen sheets on the ocean floor to crack and release tons of methane from the crust, which is a huge irreversible negative feedback loop.

It's similar to arctic permafrost melting which causes the methane stored in the frozen bogs in Russia to escape. Methane is more than 80x worse than CO2 for climate heating.

edit: funnily enough I was just looking at the novel The Deluge by Markley that covers a similar event.