Israel has started pumping water from the Mediterranean Sea into Hamas’ underground tunnel system in Gaza, The Wall Street Journal reported Monday, citing U.S. officials briefed on the matter. The flooding began sometime last month, according to the Journal. Two U.S. officials confirmed the report to ABC News, but cautioned that only some of the tunnels were being flooded, with the IDF looking to compare the tactic’s effectiveness to other strategies, which have previously included airstrikes and the use of explosives, drones, and robots. Herzi Halevi, the IDF’s chief of staff, said last week that flooding the tunnels was “a good idea, but I won’t comment on its specifics,” according to The Times of Israel. A spokesperson for the Israeli defense minister declined to comment to the Journal on Monday, saying that tunnel operations were classified.