In 1901 a mining engineer named J.B. Watson was said to have dropped plumb bobs down two 4250 foot mine shafts spaced 3200 feet apart. His measurements indicated that the plumb lines were farther apart at the bottom than than they were at the top. In other words, they diverged as they descended. Common sense would tell you that the lines would converge as they descended, because the lines should point towards the center of the Earth.
For the last century, some people like to point to the Tamarack Mines experiment as proof that the Earth is hollow.
Donald E. Simanek, who writes for MAKE magazine about curious physics (here's his article about perpetual motion that appeared in Vol 9), has an excellent article on his website that recounts the history of the alleged experiment, and examines the different frequently-offered reasons why plumb lines might diverge like this.
It's not just the Hollow Earthers who have a problem with the
prevailing theories. Here is an article I posted a while ago people
who thought the earth was flat, or perhaps wavy. There's 5,000 bucks
in it if you can prove them wrong. Pity we didn't have satellite
photos in 1931. Link