Conventional scuba systems have some major limitations. Divers using them must carefully monitor the depth and time they stay underwater and endure a series of lengthy decompression steps during resurfacing. Rebreathers recycle air, allowing divers to go deeper and remain underwater for longer, with shorter decompression on ascent. The Navy has used the devices for decades, but they were very expensive, and difficult to maintain and operate. In 2008, VR Technology introduced the Sentinel, a $12,000 rebreather with automated safety systems and full manual backup. Scientists from Woods Hole Oceanographic Institution in Massachusetts and the National Museum of China in Beijing are already using it. This July, Hollis Gear will release the VR-designed Explorer, an even less expensive ($5,400) model made for recreational divers.