Why does mud form in Sim City and more importantly how can I get rid of it?

Question

I'm playing Sim City DS, which is based on Sim City 3000 if I am not mistaken.

Answer 1

if you lay zone, but no one has moved there (no demand for that zone, no access from streets [like 3 spaces from a street] or no electricity) then when you go into build mode and use the question mark to see what it is, it will say mud - until some sims build stuff there

Answer 2

When your power plant dies it turns to mud. Do not build another power plant there or it too will turn to mud in a couple of years. I don't know why it turns to mud but it does.