The study of the universe using TeV gamma-ray photons, which are about 1 trillion times more energetic than visible light, is a rather exotic branch of astronomical science. Some of the known cosmic TeV gamma-ray sources are pulsar wind nebulae, active galactic nuclei and supernova remnants. Our current knowledge of the nature of these objects and how they produce TeV gamma-ray photons are direct results of astronomical measurements of the TeV photons that these objects produce. However, the number of photons that reach earth from cosmic gamma-ray sources are very low. Therefore, a large effective area and a high duty cycle are key features to consider in building TeV survey gamma-ray observatories. The detectors with a large effective area and high duty cycle have a greater probability of detecting the
TeV gamma-ray photons. Water is a cost effective material to use as the detector medium for detectors with large effective areas. The Milagro gamma-ray observatory was the first ground based gamma-ray observatory to use water to detect cosmic gamma-ray sources. To detect cosmic gamma-rays Milagro used water Cherenkov light, produced by relativistic charged particles in Extensive Air Showers . Milagro was able to maintain a duty cycle greater than 90%. The successor of Milagro is the High Altitude Water Cherenkov (HAWC) observatory, which is currently under construction at Sierra Negra in Mexico. When HAWC is completed in 2015 it will be an order of magnitude more sensitive than Milagro. In this talk, I will discuss the working principle of Milagro and HAWC, technical challenges of HAWC, and some physics results from Milagro and HAWC.