How do I get to the hidden levels?

By looking at the source, I've found a couple of 'hidden' levels, but not how to get to them. They are as follows:

desert
seas
Cow level

How do I get to them?


The desert can be reached by using a Get me Out of Here (G.M.O.O.H.) potion. The things you can possible end up in by using a G.M.O.O.H.-potion are:

The peaceful forest
CowLevel
sea
desert

They have an equal chance of happening.