As we're showing you abandoned places to explore in the Susquehanna Valley, some of the sites are easier than others to spot.News 8's Matt Barcaro takes you to a treasure in Lancaster County that's hidden under a hill. You can only see parts of it because the rest has been sealed off."It's unique. Geologically, it's fantastic," John Coolidge said.Coolidge has not only been to the hill in Pequea Township. He's been inside it."Then it bells out into these chambers where they mined down into the ground, went down 40 feet. Now they're filled with water," he said.Years ago, as a cave explorer, Coolidge spent a lot of time inside what was an old mine. It was started first by Native Americans hundreds of years ago, then taken over by settlers.Running through the mine are veins of quartz. You can see some of it is still exposed. In the folds of these veins, miners found some silver.During the Revolutionary and Civil wars, the mine was primarily used to find iron ore, which was turned into ammunition."They had hand tools. They had, like, a star bit. One man would turn it, and the next man would hit it with a sledgehammer. And then they'd fill it with black powder, so it was pretty serious mining," Coolidge said.After the wars, prospectors tried to get rich off of mining the silver. But by the 1930s, the mine was pretty much picked through and ultimately abandoned.The land around the mine became a park called Silver Mine Park. In the 1990s, Coolidge led tours inside the mine."The one time, I think there were over 300 people that we volunteered to take through the mine and explain it to them. They had a great interest in it," Coolidge said.The entrance is now blocked off with two internal gates and a steel plate bolted to the rock.But some visitors still can get in. A slot above the plate is a bat door made for the various species of bats that hibernate in the mine. They're the only ones still allowed to explore it.You can see both the sealed-off mine entrance and the opening that Matt climbed through at Silver Mine Park in Pequea. There's a map of the inside of the mine at the trail entrance.Abandoned PA seriesThis feat of engineering has been abandoned for nearly 130 years.Remnants of abandoned trolley line can still be seen on Gettysburg battlefield.Secret WWII POW interrogation camp has been forgotten in a state forest.Abandoned town was wiped off the map by ice jam flood.