Lublin makes a good base to explore the nether reaches of southeastern Poland and the borderlands with Ukraine. One of the best day trips, especially if you're traveling with kids, is to the city of Chelm (pronounced khelm), about 60km (37 miles) east of Lublin.
The main attraction here is an abandoned underground chalk mine, part of which has been opened to the public. With the help of a guide, you can explore around 2km (1 1/4 miles) of the mine's length. The trip is in turns chilling (quite literally since the underground temperature is a constant 48°F/9°C) and fascinating, as the mine snakes below the town and you learn how chalk was mined here over the centuries.
Chelm itself has a long history, lying at the historic crossroads of Polish, Jewish, and Russian cultures. Up until World War II, around half the city's population was Jewish. Indeed, Chelm has traditionally played an outsized role in Jewish humor, with the city's residents usually cast in the role of the "country bumpkin." Not that long ago on the Borscht Belt, a comedian starting a joke with: "There once was this rabbi from Chelm?" was enough to bring down the house.
Today, regrettably, little of this once-vibrant Jewish culture remains. The Nazis cleared the town of its Jews, and the few who survived the horrors chose to make their lives elsewhere. Recent history has been a little kinder to Chelm. The opening of the border with Ukraine has brought some economic energy to the city, and the historic center has been spruced up. In addition to the chalk mine, the city's small Old Town is a pleasant stroll and the impressive hilltop monastery is worth the slog to the top. This hill saw an early Slav settlement as long as 900 years ago.