Is it illegal if I hack a site but don't do anything?

This question has been troubling me for days. I’m into network programming and I want to see if I can take advantage of certain exploits on websites. Is it illegal if I hack them (say gain access to certain functions) but don’t do ANYTHING at all? So that I leave no trace of when/where/who/what i was doing and doing with. Technically, even if they sue me I won’t be caught, but I just don’t want to get into that trouble. Is it illegal as long as I don’t do any damage?