Pyramid — terraria

A pyramid, the top sticking out of the sand in a desert.

Two pyramids that spawned close to each other in one world.

A Pyramid is a Desert structure composed of Sandstone Bricks. Pyramids are rare, and most generated worlds will not contain any.

The bulk of the Pyramid is buried underground, with its tip sometimes protruding slightly from the Desert surface. Pyramids contain a single zigzagging tunnel leading to a treasure room, which can contain unique loot. In addition to unique treasures, they are furnished with unique decorative banners and a variety of Pots and Coin Stashes.

Notes

Rarely, pyramids will spawn in the underground desert.

It is possible, though very rare, for up to 4 Pyramids to generate in a single world. Some worlds have been sighted to have up to seven.
A Pyramid's natural tunnel sometimes ends abruptly.
An experiment generating one hundred small worlds using version 1.2.0.3.1 showed an 83% chance of having no pyramids, a 15% chance of having one pyramid, and a 2% chance of having two pyramids. However, Pyramids are slightly more common since version 1.2.3 and were made even more common in version 1.4.1.
Another experiment generating one hundred small, medium and large worlds using version 1.3.5 showed a 16% chance in small worlds, 42% chance in medium worlds and 39% chance in large worlds of having one or multiple pyramids.
Since Pyramids are made out of Sandstone Bricks which can't be corrupted, they can be helpful in blocking the spread of Evil Biomes or The Hallow.

Trivia

A second room in a pyramid found in the pre-1.3 Nintendo 3DS version.

Prior to 1.3, Pyramids were quite common and contained two rooms – one along the standard path and the other completely enclosed. The one along the path always had a Gold Chest while the second one only sometimes had one. If the second one did contain a Gold Chest, it would contain standard pyramid loot. The second room would also contain an Orange Torch and two Pyramid Banners.
While the Pyramids' real life counterparts are located in Egypt, the Cactus plants that grow in the nearby desert are based on the saguaro, a species native to the Sonoran Desert, a desert spanning large areas of Mexico and the United States of America.
Rarely, Pyramids can also spawn at beaches.^Verify