brown spots on heirloom tomatoes

[desertsol23]

We are growing heirloom tomato plants from seeds and the plants look great. However, we've noticed some of the fruit are developing brown spots on the bottom of the fruit. Any idea what this is and how to stop/prevent it?

[applestar]

There's a sticky for Spots on Tomatoes at the top of the Tomato Growing Forum. Read through it and see if that helps.
In fact, this topic really BELONGS in the Tomato forum so I've moved it there for you.