Cat and I just LOVE having a ghost town in our backyard!
The Mammoth Consolidated Gold Mine has been some what restored and is protected by the US Forest Service. There is a self guided tour available and you can take horseback rides through the area on your way to Heart Lake.
Its really beautiful here and I have alot of luck on the critter scene! Squirrels, rabbits, racoons and we've even seen a bear here once!

Years ago, when Cat first visited this mine, it was not gated like it is now. Even though the mine was (and usually is) filled with about a foot of water Cat explored it. She reports that it goes back about 1/4 of a mile and seemed very solid and stable.
Now-a-days the mine entrance is gated so exploring isnt even an option, but its still fun to explore the general area! Lots to see and truly beautiful views!

Heritage and History's Mammoth's Lake District Page
Mill City, Pine City, ect's Mammoth Consolidated Mine page
the status of the bike trail through the camp