1. Shield
   Sure
2. Shoal
   Shore

Of the 50 United States, 16 of them are landlocked and don't have any ocean shore. The U.S. state with the longest shoreline in the country isn't Florida, California or Hawaii. It's Alaska, and the state also boasts of being home to more than 3 million lakes.

1. Straight
   Stack
2. Strait
   Stream

Straight is usually used as an adjective or adverb to describe anything that isn't bent or curved, but it can also be used as a noun to define a poker hand. It doesn't define a narrow passage of water. A strait is a narrow passage of water and is sometimes known as a channel. Shown here, the Strait of Gibraltar, between the Atlantic Ocean and the Mediterranean Sea, separates Spain and North Africa.

1. Shoal
   Swamp
2. Shore
   Strait

The largest swamp in America is the Atchafalaya Basin in Louisiana, shown here. The swamp covers more than 2,200 square miles of land in southern Louisiana. The Everglades in Florida features some of the most diverse ecosystems in the world, including many swamps.

1. Sound
   Shield
2. Shoal
   Straight

The Pamlico Sound in North Carolina is one of the largest in the country. This area in North Carolina actually consists of several sounds, including the Roanaoke Sound and Croatan Sound.

1. Shield
   Swale
2. Summit
   Shore

The summit is also known as the peak, apex, acme or zentih. It's the highest point of a mountain or hill. The bottom of the mountain is known as the base and is where the mountain meets normal ground level.

1. Strait
   Swamp
2. Stream
   Sound

A stream is essentially a small river. There are several types of streams like deltas or alluvial fans. Streams are usually larger than brooks but smaller than creeks. A long and large stream is a river.

1. Spring
   Shoal
2. Stream
   Shore

Poland Spring water comes from a water spring in Poland, Maine. Some of the most popular water springs in the world are hot springs like Iceland's Blue Lagoon or the Grand Prismatic Spring in Yellowstone National Park, which is America's largest.

1. Shield
   Shore
2. Strait
   Steppe

The Eurasian Steppe, shown here in turquoise, is also known as the Great Steppe and it's the largest grassland in the world. It stretches from Bulgaria to Mongolia. In between the Mississippi River and the Rocky Mountains are the Great Plains, which is a steppe that covers half a million square miles of land.

1. Slough
   Shoal
2. Smog
   Shield

Smog is a portmanteau of smoke and fog. Smog occurs when fog or haze mixes with smoke or pollutants in the air. Smog can be deadly as it quickly spreads pollution over large areas. The Great Smog of London in 1952 and the 1966 NYC Smog are two of the most famous smog events in history.

1. Spring
   Soil
2. Shield
   Swamp

Soil is the top layer of the planet. It's made of minerals, air, water, waste, living matter, decaying matter, rocks and more. It can be seen as a layer of skin wrapped around the planet. It's where plants grow.

1. Strata
   Sediment
2. Slough
   Spit

Sediment is just very, very small pieces of solid matter that can literally be anything. When sediment accumulates and hardens, it creates sedimentary rock. The distinct layers of a sedimentary rock are called the strata.

1. Scale
   Sheepback
2. Seamount
   Sedimentary rock

A sedimentary rock is made of sediment, which is loose matter carried by wind or water and eventually deposited somewhere. When enough sediment accumulates and hardens, it can form a rock. Sedimentary rocks sometimes look like they're layered or made of several different materials.

1. Soapstone
   Slate
2. Sandstone
   Scoria

Soapstone is a metamorphic rock, which means it was created by way of transformation. Scoria is an igneous rock, which is a rock formed by the cooling of magma. Sandstone is made from sediment and its strata are often distinctly visible.

1. Shoal
   Spit
2. Slough
   Silt

Silt is usually tiny pieces of quartz about the size of a grain of sand. A handful of dry silt might feel like powder or dust and it's sometimes known as "rock dust." Wet silt, shown here, can resemble silky-feeling mud. Sedimentary rock can be made of only silt, and those rocks are called siltstones.

1. Swamp
   Sinkhole
2. Stream
   Sound

A sinkhole is a crater or depression that occurs when material below the surface is removed and the surface caves in. A pothole is caused by a defect on the surface layer, whereas a sinkhole is caused by a defect underneath the surface layer.

1. Seabed
   Seamount
2. Sea level
   Second home

The ocean floor is one of the most mysterious parts of the planet. It covers a majority of the world, but only about 5% of it has been explored. We have little to no clue what's down there and extraordinary discoveries are made every time we look.

1. Shoal
   Shield
2. Strait
   Sheepback

A sheepback rock gets its name because it resembles a sheep's back. The official name of the rock is roche moutonnee, and it has a distinct shape. The rock formations were first discovered and defined in the 18th century.

1. Strait
   Shoal
2. Sediment
   Shield

A shoal is a very shallow area of water created when sand and sediment accumulates enough to rise to the top of the water. Shoals are also known as sandbars and sandbanks. A shoal also defines a large group of fish swimming together. A school is a large group of fish swimming together in the same direction.

1. Slough
   Strait
2. Smog
   Sinkhole

Sloughs are wetlands where water doesn't flow, flows insanely slow or only flows during certain times of the season. It can also describe an inlet that isn't consistently filled with water and has no water flow.

1. Strait
   Spit
2. Stream
   Sound

Spits form when sediment deposits and accumulates enough to create a landform that projects into the ocean. One of the most famous in the world is Spurn Head, shown here, located off the coast of Yorkshire. Spurn Head is more than three miles long.

1. Swamp
   Shield
2. Spit
   Spur

If you've ever watched the show "Game of Thrones," you're familiar with spurs and more specifically spur castles. Spur castles are castles built on spurs. They are surrounded by steep cliffs on three sides and offer great natural defense.

1. Shoal
   Slough
2. Strath
   Spit

Strath stems from a Gaelic word mostly used in Scotland to describe broad valleys. A glen also stems from Gaelic and is often used in Scotland. A glen is a narrow valley and a ravine is a narrow valley with steep slopes on both sides.

1. Swamp
   Swale
2. Stream
   Slough

A swale is any shady, sunken or marshy spot, and they can be natural or man-made. Man-made swales are usually used for irrigation control and to manage runoff. Beach swales are shallow trenches between sandbars.

1. Slough
   Swamp
2. Savanna
   Second home

A savanna is a steppe near the equator. It's hotter than a steppe and drier than a forest but consists of more grassland and vegetation than a desert. The Serengeti in Tanzania and Maasai Mara in Kenya, shown here, are two of the most famous savannas in the world.

1. Stack
   Spit
2. Strait
   Source

A river's source is also known as its headwaters and it can be anything from a glacier to a lake or spring and anything in between. The end of a river is called its mouth or delta and it usually deposits into an ocean, lake or marsh. Shown here is Karst Spring, source of the river Rhume in Germany.

1. Stalagmite
   Strait
2. Stalactite
   Summit

Stalagmites are formed as water drops to the floor of a cave and material within the water accumulates and hardens. Stalagmites can be made from a bunch of things from lava to limestone and even hardened rodent urine. Lava stalagmites can form in hours while others may need thousands of years.

1. Stalactite
   Summit
2. Strait
   Slough

Stalactites are the counterparts of stalagmites and sometimes they even connect with each other. The easiest way to remember the two is that stalaCtites grow from the Ceiling and stalaGmites grow from the Ground.

1. Stream
   Swash
2. Slough
   Strath

Onomatopoeia is the process of creating words that phonetically sound like what they describe, and swash is one of those words. The swash is the last layer of beach water that washes across shore after a wave breaks. The swash usually creates a line in the sand between wet and dry.

1. Strata
   Strait
2. State
   Sound

You can distinctly see each layer of some sedimentary rocks, and these layers are called strata. One layer is called a stratum and the study of these layers is called stratigraphy. Strata can be seen all over the world.

1. Thermosphere
   Mesosphere
2. Exosphere
   Stratosphere

When you look up at the sky, the first 7.5 miles you can see is the troposphere. The stratosphere begins at the end of the troposphere and extends about another 25 miles. Jet planes don't fly higher than the stratosphere. A rocket is usually needed to break the stratosphere. Shown here, Space Shuttle Endeavour straddles the stratosphere and mesosphere.

1. Sandstone
   Shale
2. Slate
   Scoria

Shale and sandstone are sedimentary rocks and scoria is an igneous rock. They're much weaker than metamorphic rocks, which are usually created under intense levels of heat and pressure. Slate can be formed in several colors from gray to purple to blue.

1. Snowline
   Sound
2. Soil horizon
   Slough

Some elevations are so high they're always covered in snow. These coverings on mountains are called snowcaps. The bottom of a snowcap is the snowline. It's the lowest altitude where snow doesn't melt, no matter the season. Snowlines can rise and fall with the seasons, but they never disappear.

1. States
   Steppes
2. Stacks
   Spurs

Sea stacks are some of the coolest geological features on Earth and usually create awesome vistas. The largest stack in the world is Ball's Pyramid, shown here, in the Pacific Ocean near Australia. The limestone stacks in Vietnam's Ha Long Bay and and the Twelve Apostles near Australia are some of the world's most famous.

1. Slough
   Scarp
2. Shield
   Shoal

Scarp is short for escarpment and it usually forms after a piece of the mountain falls off or extreme erosion creates a sheer cliff. Some of the most famous sheer cliffs in the world are Ireland's Cliffs of Moher, Canada's Mount Thor and California's El Capitan, shown here.

1. Shield
   Slough
2. Saddle
   Seif

Seif dunes are also known as longitudinal dunes and they can be up to 100 miles long. They have two steep faces on each side, and that creates a sharp crest at the top of the dune. Seif dunes run in the direction of the wind. Shown here are seif dunes in the Great Sand Sea in southwest Egypt seen from the International Space Station.