Carbon Dioxide Sequestration Gone Wrong

I'm sorry but I can't help thinking that pumping liquefied carbon dioxide into the ground is a bad idea waiting to go horribly, horribly wrong. From reading about the idea it sounds like the only problem with the CO2 leaking to the surface is a carbon credit lost, but what happens when 100,000 tons of denser than air CO2 reaches the surface, perhaps catastrophically one day? I really don't want to be around to find out because its gonna hang around near the ground and start killing people. If you think this is far fetched you should read about Lake Nyos, the "bad lake" where more than 1,700 people died from CO2 poisoning. The CO2 came from underground sources - and rolled out over a 10 mile radius killing all it encountered. Now does that sound familiar?