From Tunisia to Libya

Hi !

I am a Spanish guy who wants to travel to Tunisia next summer (I know, it will be host, but I don't have choice). My main objective is to make trek in the desert in the south side of the country, the if it is possible why not to go to Libya for fews days just to discover this not very known country..

Is it possible to make the trip by Bus or train ? How much does it costs ? Is it a safe trip ? Where can I go in Libya for a short stay ?

Thanks for your recommandations ..

P.