"We believe they formed as a result of repeated warming periods. These periods impacted hydrate in the deep ocean and the released methane migrated up to 40km t

"We believe they formed as a result of repeated warming periods. These periods impacted hydrate in the deep ocean and the released methane migrated up to 40km towards the continent, to be vented beyond the shallowest hydrate deposits." So we're talking a geological process, on a geological timescale, on one hand. On the other hand, I've always heard that water is densest at 4C, which is probably the temperature at the bottom of any ocean, regardless of the warmth of surface water (which is what has been in the news). So, what's up with the headline? I read the whole piece, and it still doesn't gibe.