Is Dwarf Fortress open source?

I'm curious to know, is the popular game Dwarf Fortress open source? I've heard a lot about its unique gameplay and intricate world-building, and I'm wondering if the source code is publicly available for anyone to view, modify, and contribute to. If so, that would be quite fascinating and could potentially open up a whole new world of possibilities for the game's community. Could you shed some light on this for me?