My friends and I recently visited this abandoned mercury mine. The site was cleaned-up by the EPA many years ago, but much of the old equipment remains. Up the hill from the equipment are several deep cuts in the earth and a humongous hole in the ground. The sediment at edges of this hole are VERY unstable and are ready to fall in, take extreme caution! The hole is quite deep and tunnels can be seen extending off in several directions. 

There are many ropes, and ladders along the way that are there to help assist you up and down into the mine. If you feel confident that you're strong enough to hold up your own body weight then you should be fine. If not, make sure you have someone there to spot you. Also - If you are terrified of heights then this may not be for you. Now once you've made your way down into the mines there are many caves and mine shafts to explore and if you're lucky you might just find the geocache at the end of one. Make sure to write a thoughtful message to future adventures in the notebook ;)

Remember that you MUST have a headlamp if you want to be able to see where you are stepping... and I would highly recommend exploring with a group of 3 or more people. 

Have Fun!