ground-effect machine: see air-cushion vehicle.

ground squirrel, name applied to certain terrestrial rodents of the squirrel family. In North America the name refers to members of the genus Citellus and sometimes to the closely related genera Tamias (chipmunk), Cynomys (prairie dog), and Marmota (marmot and woodchuck). Citellus species are found in Asia, E Europe, and North America. In the Old World they are called sousliks. Other ground squirrel genera are found in Africa and S Asia. The approximately 30 North American species of Citellus are found W of Hudson Bay, from the Arctic Ocean to central Mexico. These ground squirrels have rounded heads, short ears and legs, and shorter, less bushy tails than tree squirrels. Their combined head and body length is 41/2 to 131/2 in. (11.4-33 cm) depending on the species; the tail is usually a third to two thirds as long. Most are gregarious, living in extensive underground burrows; they hibernate in colder parts of their range. Members of different species are found in prairie grasslands, arctic tundra, mountain meadows, open forest, desert, and scrub country. In some regions the ground squirrel is called gopher, a name more commonly applied to burrowing rodents of a different family. Primarily vegetarian in their diet, ground squirrels may become agricultural pests, but they destroy insects and mice as well as crops. Their tunnels cause landslides and erosion, but also serve to mix and aerate the soil. Ground squirrels are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Sciuridae.

ground speed indicator: see air navigation.

ground pine, common name for several creeping plants, particularly species of club moss.

ground laurel: see trailing arbutus.

ground ivy, trailing perennial herb of the genus Glechoma of the family Labiatae (mint family), closely related to catnip and naturalized from Europe. It forms a dense ground cover and spreads rapidly, thriving in cool, damp places. Its leaves were once used to brew a cough remedy; in quantity it is reputed to be poisonous to horses. Gill-over-the-ground is another of its many names. Ground ivy is classified in the division Magnoliophyta, class Magnoliopsida, order Lamiales, family Labiatae.

ground bass, melodic phrase used repeatedly as a bass line. In its earlier form, developed in the 13th and 14th cent., the ground or basso ostinato [Ital.=obstinate] never varied in harmonization or pitch. The tenor, or pes, of Sumer Is Icumen In is such a ground. Another sort was developed during the 17th cent. by Purcell and his contemporaries. This ground was not rigid as to pitch, sometimes moving from bass to soprano. It was composed with varying melodies and harmonies in the upper parts. The result was often a series of variations as in the baroque chaconne and passacaglia. The device often has great dramatic effect. J. S. Bach and Handel made remarkable use of it.

or hovercraft

Vehicle supported above the surface of land or water by an air cushion, produced by downwardly directed fans, enclosed within a flexible skirt beneath the hull. The concept was first proposed by John Thornycroft in the 1870s, but a working model was not produced until 1955, when Christopher Cockerell solved the problem of keeping the air cushion from escaping from under the vehicle, and formed Hovercraft Ltd. to manufacture prototypes. Problems with skirt design and engine maintenance have restricted the vehicle's commercial application; today hovercraft are used mainly as ferries.

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Complex formerly consisting of seven buildings around a central plaza, near the southern tip of Manhattan. Its huge twin towers (completed 1970–72) were designed by Minoru Yamasaki (1912–86). At 1,368 ft (417 m) and 1,362 ft (415 m) tall, they were the world's tallest buildings until surpassed in 1973 by the Sears Tower in Chicago. The towers were notable for the relationship of their simple, light embellishment to their underlying structure. In 1993 a bomb planted by terrorists exploded in the underground garage, killing several people and injuring some 1,000. A much more massive attack occurred on Sept. 11, 2001, when first One World Trade Center and then Two World Trade Center were struck by hijacked commercial airliners that were deliberately flown into them. Shortly thereafter both of the heavily damaged towers, as well as adjacent buildings, collapsed into enormous piles of debris. The attacks claimed the lives of some 2,750 people. Thousands more were injured. See September 11 attacks.

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Any of numerous relatively short-legged, terrestrial rodents of the squirrel family (Sciuridae), found in North America, Mexico, Africa, Europe, and Asia. The name is often applied to chipmunks. Ground squirrels belong to the genera Ammospermophilus, Xerus, Atlantoxerus, and Spermophilus. They live in burrows, sometimes in colonies. Though primarily herbivores, some feed on insects and other small animals and on carrion. Many species collect food, carrying it in their cheek pouches, and store it in their burrows. Those in cold areas may hibernate in winter; those in dry areas may become dormant in summer. Species range from about 7 to 20 in. (17–52 cm) in length, including the tail.

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Name that originally referred to a species of southern European spider (Lycosa tarentula) but now refers to more than 175 spider species (family Lycosidae) found in North America, Europe, and north of the Arctic Circle. The body of L. tarentula, the largest species, is about 1 in. (2.5 cm) long. Most species have a long, broad, hairy brown body; stout, long legs; and strong, prominent jaws. Wolf spiders chase and pounce upon their prey, hunting mostly at night. Most species build a silk-lined, tubular nest in the ground, which they dig with their heavy front legs. A few species spin webs. The bite of L. tarentula produces no ill effects in humans.

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Any of about 200 species of primitive vascular plants that constitute the genus Lycopodium (order Lycopodiales), mainly native to tropical mountains but also common in northern forests of both hemispheres. They are evergreen plants with needlelike leaves and, often, conelike clusters of small leaves (strobili; see cone), each with a kidney-shaped spore capsule at its base. Representative species include running pine, or stag's horn moss (L. clavatum), ground cedar (L. complanatum ‘flabelliforme'), shining club moss (L. lucidulum), fir club moss (L. selago), ground pine (L. obscurum), and alpine club moss (L. alpinum).

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or mayflower

Trailing evergreen plant (Epigaea repens) of the heath family, native to sandy or boggy, acidic woodlands of eastern North America. Its leaves are oblong and hairy, and its white, pink, or rosy flowers grow in dense clusters. It is grown in shady wildflower gardens and as ground cover in terraria.

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or blue ground

Dark, heavy, often fragmented igneous rock that may contain diamonds in the rock matrix. Kimberlite is a mica peridotite, an ultrabasic rock type with a complex and often highly altered mineral composition. It occurs in the Kimberley district of South Africa and the Kimberley and Lake Argyle regions of Australia, as well as near Ithaca, N.Y.

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