Near Paragon. ATV's ride there, has a donation box. No Trespassing signs abound.

Thanks to Barry Welling for the tip!

New insights on Illinoian deglaciation from deposits of Glacial Lake Quincy, central Indiana

This looks like a reasonable approximation of the glacial Porter Cave siphon:

From http://www.in.gov/isdh/22926.htm#Porter Cave and Campground

Porter Cave and Campground
Drunkards Pike Road
Paragon
39 Dependent sites
0 Independent sites

Water: Public
PWSID: N/A
Sewage Disposal: Unknown
Pool: No
Beach: No

Owner:
Christian Tabernacle Church
Joseph Kent Jordan
802 South Franklin Road
Indianapolis IN 46219
Phone: