BARNSTABLE - Dressed in black rubber suits, 11 men and one woman stood on 7 inches of ice in the middle of Hathaway Pond. Next came the chainsaw.
They came looking for a winter thrill, and found it in ice diving. Common among research scientists in Antarctica, the sport is steadily becoming popular with recreational divers. Like ice fishing, for those who long to reel in a catch during the winter, ice diving attracts scuba divers who aren't ready to hang up their gear because of freezing temperatures.
There has been a 40 percent increase in the number of people seeking recreational ice diving certifications since 2002, according to the Professional Association of Diving Instructors.
''I like snowboarding, skydiving, and winter camping, so how could I not go ice diving?'' said Robert Granetz, a 52-year-old nuclear research scientist at MIT. Granetz, who lives in Reading, completed his ice diving certification two years ago and came this afternoon, with other assistants, to help instructor Steve Brown of Walpole with seven students ready to take the plunge.
To be eligible for an ice diving certification from PG Dive in Newton, which ran this trip, divers must already have a scuba certification and attend a four-hour lecture. The practicum, which takes place over two days and costs $259, requires each participant to make three 20-minute dives and act as the tender - an above-ground human anchor for the tethered diving pairs. (In the research science community, the entry-level scientific ice diving course takes 100 hours and includes 12 training dives, a four-hour lecture, two three-hour pool-training sessions, and 12 open-water training dives.)
The day began with Brown cutting a 10-foot triangle into the ice, tying a rope around the end, and sliding the large cutout underneath the surface where it remained for the day (it was later pulled back into place to re-freeze). In groups of two, the divers slid off the edge of the triangle and slipped into the 37-degree water, which was 5 degrees warmer than the air. The divers were tethered with a 100-foot rope attached to their equipment, and held by a tender standing at the edge of the hole. Next to the tender stood a rescue diver with a 200-foot bright orange rope, prepared to help out in the rare case that a diver became detached.
The water was clear, but two years ago, during a certification in New Hampshire, divers complained about poor visibility. ''Two feet away from the hole and you couldn't see,'' said Peter Richardson, a 40-year-old software manager who lives in Brookline. ''It was a lesson in panic control.''
Angst aside, divers marvel at the beauty of life under the ice, though there aren't many fish to see this time of year. ''The bottom side is usually smoother than the top, and you can see your reflection,'' Brown said. ''Sometimes things are frozen in there like a branch, and there are cracks and bubbles.''
Ice divers wear dry suits, not wet suits. A wet suit works by trapping a small amount of water between the neoprene rubber and the skin, which heats up and acts as an insulator. A dry suit is sealed at the neck and wrists so no water can get inside, which is important when the temperatures are this cold. And unlike a wet suit, a dry suit must be pressurized as the diver
Still, the fact remains: It was freezing outside. A few hundred feet from the diving hole was a tent for the divers to change in. Surprisingly it was a good 10 degrees warmer inside. Next to the tent sat a pot of water, kept warm by a camping stove, used to thaw frozen equipment. Another pot of hot water was for hot chocolate, tea, and instant soup.
More concerned about claustrophobia than seeing wildlife, Matt Silvia of Quincy was happy that his nerves held out. ''When you look up through the ice it's like having a crystal ceiling,'' said Silvia, a 34-year-old software engineer and shipwreck diver. ''It's really gorgeous.''
The spring-fed kettle pond in Barnstable runs 60 feet deep. In one area lies a 30-foot shipwreck put down 20 years ago by the local fire department for diver training. There's also a sunken Saab automobile that had been stolen and driven out on the ice, where it fell through.
''It's a very challenging form of diving,'' said Derya Akkaynak, the only woman getting certified that afternoon. The 25-year-old information technology consultant recently finished her master's degree in aeronautics and astronautics at MIT. She said she was concerned how she would fare with the harsh temperature while under the ice, but she said she ended up being ''very comfortable.'' She was all smiles after she emerged from the water.
''It's probably the safest form [of diving], as everyone's connected to a line and a buddy, but my parents would be worried [if they knew],'' she said as she warmed up inside a big cooler filled with hot water. Akkaynak, a native of Turkey, did not tell her family there that she'd be spending the weekend under 7 inches of ice.
''I'm planning on telling them in a couple of years, when they can't make a big deal out of it anymore,'' she said.
Susan Chaityn Lebovits can be reached at Lebovits@globe.com.