At 45 miles long and 28 miles wide, the Yellowstone Caldera is a sort-of reverse mountain. In some places you can see the distinct drop off from the Caldera rim into the basin that makes up most of Yellowstone.
Two hiking trails lead to the top of Mount Washburn, which sits at 10,243 feet.
Gallatin Mountain Range
The Gallatin Range sits at the northwest corner of Yellowstone National Park. It's easy to access via many trailheads along US 191 between West Yellowstone and Big Sky Montana.
Beartooth Mountain Range
In the norhteast corner of Yellowstone the Beartooth Mountain Range is easily accessible via the famous Beartooth Highway, between Cooke City at the Northeast Entrance and the town of Red Lodge Montana.
A small mountain range towards the southern end of Yellowstone, the Red Mountains encompass Mount Sheridan and Factory Hill. Heart Lake is a great hiking trail that offers access into the Red Mountains.