Soda Slurpee

The greenhouse effect is generally thought to be a bad thing, and CO2 is thought to be the big baddy in the process.

Underwater, at depth gas has a tendency to create hydrates - ice with gas in it. These hydrates sit on the ocean floor and generally have a pretty lazy time of it not doing much apart from maybe proping up another bit of the ocean floor.

This idea is to send CO2 down to the ocean floor and create hydrates. This is where the groovy bit comes into it. By using vehicles on the ocean floor (see my other idea Hooversub, link provided in a shameless self promoting way) the CO2 hydrates can replace the hydrates removed. Technically they'll be proping up any potentially unstable sections of the ocean floor and so it wont be dumping CO2 at sea as by international law you're not allowed to dump at sea, however as a side benefit we get to remove tons of carbon from the atmosphere.

I'm not sure, but I believe CO2 might be a by product of the liquid air industry, however that wouldn't provide enough to impact the concentrations in the atmosphere so it'd be neccesary to set up scrubbers to provide enough CO2, conveniently enough there's enough methane down there to provide the power to do it.

The CO2 could be shipped down to the subs in the tanks that the methane is shipped ack up in.