An entertaining and informative alternative to watching and re-watching old movies is science and nature videos. National Geographic Video, a leader in this field, this week released three more in its Specials and Explorer series. A double dip is in store for adventure and science buffs next month when Vestron releases the first three titles in its Nova Video Library.

The choices will be plentiful this year. Lorne Greene's "New Wilderness" volumes are scheduled for a dozen new titles that were assembled before his death, and Lorimar is releasing six more in PBS' award-winning "Nature" series.

The new National Geographic releases are eye- and mind-filling, every bit as captivating as the previous 19 titles. They are available at National Geographic (921-1200 locally and 1-800-638-4077) if your neighborhood outlet doesn't have them. All are 60 minutes long and priced at $29.98 (Vestron):

The Grizzlies: A fascinating portrait of this powerful, intelligent and short-tempered creature from Yellowstone Park to the ranges of Alaska, with lots of breathtaking footage and facts. These big brown animals grow to about a ton, but weigh a mere pound at birth; they can dash 50 yards in 3 seconds (move over, Darrell Green) and can pick up the scent of food a couple of miles away (80 percent of their diet is made up of berries, nuts, roots and greens). But where there used to be 50,000 grizzzlies, now only about 900 are left in the lower 48 states, more than 200 of those in Yellowstone.

Himalayan River Run: For those who have seen enough of people climbing toward the peak of Nepal's Mount Everest, this cassette offers the flip side: Five men drop more than 14,000 feet while negotiating the turbulent and icy white waters of the Dudh Kosi (milk) River. Three tons of equipment had to be carried more than 100 miles before the descent could begin. At one stretch the men and their kayaks battled a stretch of 80 miles of white water that fell more than 11,000 feet. They spilled through the rocks and gorges so rapidily it seemed as if they had no time to think. The journey, covering 450 miles, took eight weeks.

Ballad of the Irish Horse: Ireland's horses, wild stallions, thoroughbreds and work ponies are still a vital part of Irish work, play and history. The climate and rich emerald grasses lend themselves to splendid photography. The devotion of the Irish to the horse -- which in this video seems to be part of the landscape -- is amazing. It was in Ireland that steeplechase racing was born, when in 1752 two men raced horses across fields and over fences from the church steeple in one town to the steeple in another. Show jumping also originated in Dublin.

"Nova" is the longest running and most popular science documentary series on American television. The show has received almost every major broadcast industry award since its debut in 1974. Nearly 300 "Nova" programs have been produced by WGBH in Boston, giving the Nova Video Library project ample choices for its new venture. The project's first three offerings are scheduled for release March 30 (all 60 mins., $29.98, Vestron):

Visions of the Deep: Al Giddings, master of underwater photography, shows and discusses gems from his film library, which may be the world's most valuable collection of underwater photography. Scenes include Tahitian children playing piggyback with sharks, divers feeding and fondling moray eels, the awesome cobalt blue waters under the polar ice cap, and face-to-face shots of 3,000-pound great white sharks off the southern Australian coast. There's also a look at the lagoon at Truk where the United States sunk some 70 Japanese ships that have evolved since World War II into majestic coral-laden reef communities. Michael Landon narrates.

Einstein: A most scientific documentary about this genius who believed that man and his fate should be the center of scientific research. Home movies, newsreels and photographs are artfully spliced to portray Albert Einstein and his battle for international peace and nuclear disarmament at the outset of the Cold War. It's surprising to learn that he failed his first university entrance exam.

UFO's: Are We Alone?: Several reported incidents of unidentified flying objects are explored, without any conclusions being offered. Serious study of this mysterious phenomenon is very much alive, and the program suggests that significant discoveries may not be far off. Apollo astronauts talk about being shadowed by mysterious craft on their way to the moon, a BBC crew films a UFO over New Zealand, and an Arizona lumberjack tells of being abducted by aliens and taken into their space ship -- and he passes a lie detector test.

In June, "Nova" will release three new shows, with plans for three more every three months. Videos scheduled for June release: Case of the Ancient Astronaut; Sign/Apes, Song/Whales, and Fat Chance in Thin World.