The largest manmade cave in Israel was found 3 miles north of Jericho, and may have been used as a monastery in later years. From the Jerusalem Post:
An artificial underground cave, the largest of its kind in Israel, was discovered in the Jordan Valley during excavations by the Haifa University’s Department of Archaeology. Prof. Adam Zertal, who headed the dig, assessed that the cave was used as a quarry in the Roman era. Various carvings were found on the cave’s walls, including some of crosses, leading to the notion that the cave might have also hosted an ancient monastery. The cave, sprawling over four dunams [1 acre] ten meters [32 feet] under the face of earth, is located some four kilometers [2.5 miles] north of Jericho. It was discovered at the end of March 2009 as part of a Haifa University dig which began in 1978, and is the largest man-made cave ever uncovered in Israel. The cave’s main hall is supported by 22 pillars, on which are engraved 31 crosses, a zodiac-like symbol, roman numerals and a Roman legion’s pennant. Judging by the findings, Prof. Zertal dated the cavern to around 1 CE. "Initially, the place was utilized as a quarry, which was active for 400-500 years. But the other findings definitely give the impression that the cave was used for other purposes, such as a monastery, and perhaps even a hideaway," said Zertal.