Down the hole

[CookieThief]

So I was mapping out my local underground (literal not musical) and I came across this hole. Its is part of a Storm drainage system and its about the size of those tunnel slides at playgrounds. The problem is that It is like a slide and I don't know whats on the other side. I tossed some rocks down and I couldn't even hear it hit the bottom. I have dealt with manholes and the like but this one gives me a bad feeling. The urban explorer in me is curious but it feels off. Any advise?

[Xanatos]

Glowstick on the end of a string. Lower it down and 'fathom' how far it goes; one fathom is about 6ft, or about the length of your outstretched arms. Get a decent length of string, lower it down the hole with a weight on the end of it (preferably a glowstick so you can see how far down the hole goes) and pull it back up by stretching the string between your outstretched arms, measuring how many fathoms it takes to retrieve it.

Bit hard to explain in words, Lindybeige has a video on rope where he explains what 'fathoming' is: https://www.youtube.com/watch?v=lZfpQzV2GnM
(fathoms bit starts at around 3:06)