As a portable light source, only the LED flashlight is superior to the carbide miner's lamp, which had been the standard for brightness, weight and reliability since the early 1900s. Carbide lamps have an upper chamber full of water and a lower chamber full of rocklike calcium carbide (CaC2). A valve lets the water drip onto the CaC2 at a steady rate. When water hits carbide, it produces acetylene gas, which is directed to a nozzle in a parabolic reflector and then lit manually. Burning acetylene is extremely bright, and a cave explorer can keep a light going for days with just some water and a bag of what look like ordinary stones.