First, BP unsuccessfully attempted to close the blowout preventer valves on the wellhead with remotely operated underwater vehicles.[72][73] Next, it placed a 125-tonne (280,000 lb) containment dome over the largest leak and piped the oil to a storage vessel. While this technique had worked in shallower water, it failed here when gas combined with cold water to form methane hydrate crystals that blocked the opening at the top of the dome.[74] Pumping heavy drilling fluids into the blowout preventer to restrict the flow of oil before sealing it permanently with cement ("top kill") also failed.[75][76]