On episode 3 of season 3, they described how they were going to go looking for the Hedden shaft, which was dug by Gilbert Hedden, in 1937. It ran parallel to the money pit to a depth of 125 feet, then joined into it.

But now, the exact location of the Hedden shaft and the money pit, are uncertain. They were covered over with dirt and vegetation grew over top. So the episode ended with the team beginning to scrape off the top dirt, to hopefully find the Hedden Shaft again.

For my future reference, here’s that NMR map with proposed underground tunnels, shown in relation to the area of the money pit and 10X.