Though I don't have any pictures, Dae Gaeum Cave is located across the valley from Hwanseon Gul Cave. The two caves are completely different from each other- where the Hwanseon is a large, mostly dry cave, Dae Gaeum is a smaller, almost completely filled with flowing water cave. The Korean stainless steel walkways follow the waterways up to the largest and best looking sump I saw in Korea, perhaps 50 feet wide, and appeared to be 20-30 feet deep. There was a large line running into the sump, and I heard rumor that there was 1500 meters of passage beyond the sump, but that is unconfirmed. Visiting Dae Gaeum is a little more difficult, as it is not geared towards non-Korean speaking tourists. Despite this, it is fairly easy to get a ticket to the tram (the little tram drives right into the cave, there is no other way in.)
Photo stolen from the Korean Tourism Website.
You can visit the Korean Tourism website for this cave here:
http://english.visitkorea.or.kr/enu/ATR/SI_EN_3_1_1_1.jsp?cid=789825