According to Environment, Health, and Safety Online, batteries can cause many environmental hazards such as:
- pollute bodies of water when metals vaporize into the air when burned
- contribute to the metals that leach from landfills
- expose the environment and water to lead and acid
- cause burns and danger to the eye and skin
So, what can you do with your old batteries? On the same site I listed above, there is a big table with all types of batteries and what you should do to dispose them. Some of the options include recycling, hazardous waste collection sites, etc.
Also, I thought I should mention rechargeable batteries. Though these are still made of the same metals and materials, they obviously can be used many times and therefore if you make use of them you will not have so many batteries to try to get rid of! We have some at home and it's quite easy to charge them up, actually - just stick them in the charger for a while until the light turns green. But make sure not to mix them with regular batteries!
This image is from Bristol Batteries.