Easier, but not safer. Pure carbon (mixed with something inert, to prevent burning) doesn't need armed guard for the next 500 years at least. IMHO, the easiest

Easier, but not safer. Pure carbon (mixed with something inert, to prevent burning) doesn't need armed guard for the next 500 years at least. IMHO, the easiest and cheapest way to store carbon is to grow algae to make hydrocarbons (oils), separate them mechanically, then spread them over a soil as fertilizer, or pump them underground, or burn part of the algae to make a kind of charcoal (to prevent biodegradation).