If you mean "bottom" as the end opposite the stem end, it's most likely blossom end rot. The plants ran short of water during the tomato's development.

If you mean "bottom" as the part that's resting on the soil, most likely physical scarring/brusing and/or the fruit resting on the moist soil surface.

Blossom end rot is caused by a calcium deficiency. The plant is not able to access the calcium in the soil. It usually happens early in the growing season when the soil is cool.

Here's a link to more information on the Tomato Forum.

Here is a link that might be useful: Blossom end rot on tomatoes

Blossom end rot is caused by a calcuim deficiency....yes.....but the calcium deficiency is caused by lack of water.
Keep your tomatoes evenly moist and you won't have a problem.
You will notice it occurs most often in potted tomatoes.
Linda C

Here is a link that might be useful: blossom end rot

I put straw under them to keep them off the damp ground & throw around some coffee grounds to deter slugs.

Best luck!