Future cities could include pancake-shaped buildings, power plants that harvest lightning and ocean-based skyscrapers that produce potable water and clean up trash. Those are some of the visions in the 2011 eVolo Skyscraper Competition, a forum for futuristic — and even fantastical — ideas for new architecture.

Click here to see the winning designs and some other interesting entries.

Hosted by the architecture magazine eVolo, the competition is meant to stimulate discussion, development and promotion of new concepts for vertical density. Participants are asked to examine the relationships among the skyscraper and the natural world, the community and the city.

The top three awards went to designs that focus on the environment, whether it’s through cleaning polluted air or re-imagining one of the marvels of the modern world, the Hoover Dam. A host of honorable mentions include environmental cleanup facilities, sustainable communities and even subterranean communities for the living and the dead.

Click through to the photo gallery for some of the highlights from this year’s competition.

First Place: A Ferris Wheel Greenhouse Made of Recycled Cars

The winning design would be based in New Delhi, one of the dirtiest cities on Earth, and incorporates a wind turbine into a circular greenhouse that would help clean the air. The Ferris-wheel-shaped building is made from recycled cars, and waste heat from the on-site recycling center would be used to grow plants to produce biofuels.

Second Place: Flattened Tower

Not everyone wants the tallest skyscrapers money can buy. Some cities, like Washington, D.C., don’t even allow them, because they would interrupt important landmarks and ruin pretty skylines. This flattened approach would provide some of the benefits of vertical living while keeping things relatively close to the ground. Holes in the structure allow sunlight to reach the ground, and its wide berth would enable large solar arrays or rainwater collection systems, eVolo points out.

Third Place: The Hoover Dam, Reimagined

Yheu-Shen Chua of the United Kingdom designed this update to the Hoover Dam, turning the concrete behemoth into a curvy structure evocative of water-carved canyon rock. Instead of a flat viewing platform, the new structure would engage visitors with the water, incorporating a vertical aquarium and other amenities.

Honorable Mention: Tower For the Dead

Designed for Mexico City, a place that already celebrates its dead with unrivaled fervor, this subterranean tower descends into the Earth like something out of Dante, and is designed to help people grieve. It rises slightly above the ground, and at the pedestrian level it looks like an empty hole — “the sensation of vacuum represents the absence caused by death,” according to its Mexico-based designers. Mourners would descend along with the casket of their lost loved one, and they would be “reborn” into the light after their trip to the underworld. The tower also solves the problem of where to bury the elderly, especially in overpopulated areas like Mexico City. After a parting ceremony, the caskets are taken to a crematorium.

Honorable Mention: Sports Complex

Why build a separate football stadium, basketball arena and ballpark, when you can combine them all into one structure? Sports Tower, designed by a Ukrainian architect, would let visitors move between various venues, so you wouldn’t have to choose between spring baseball and the NBA playoffs. It could conceivably host the soccer World Cup, Olympics events and tennis grand slams. Then it would just be a battle over who gets the megacomplex naming rights.

Honorable Mention: The Sixth Borough

This tower would occupy the space between 22nd and 14th streets and 6th and 7th avenues in New York City, towering over the existing infrastructure. A park sandwiched between the residential spaces and office towers gives residents a calming oasis, and everything would be connected to the subway system.

Honorable Mention: Hydra Power Station

Built from a graphene exoskeleton, the Hydra skyscraper is designed to harvest power from lightning and store it in giant batteries. Hydra would power hydrogen fuel cells, whose byproduct is clean water. The project also includes living quarters for scientists and their families.

Honorable Mention: Seascrapers

Oil rigs could be repurposed as water-purification facilities, storing desalinated water for delivery to thirsty countries. These “seascrapers” would involve modified oil pipes that can bring water into spherical containers, where it would be distilled and desalinated.

Honorable Mention: Lady Landfill Skyscraper

This concept takes advantage of 3.5 million tons of garbage to create floating iceberg-like islands, which would serve as garbage aggregators and recycling facilities. It would use trash in the Great Pacific Garbage Patch, estimated to be twice the size of Texas. The islands’ goal would be to collect and process the trash, also using part of it to generate energy. They would also have housing and recreational facilities for workers.

Honorable Mention: Rhizome Tower

Like the plant root systems for which they are named, Rhizome Towers would be mostly underground, but partly exposed to the light. They could be used to construct entire underground cities, in response to climate change or global catastrophe. The first layer of the “groundscraper” would be above the surface and host recreational and agricultural facilities, along with solar panels. The 60-level second layer would be for living quarters, including a variety of house types and sizes; the third and fourth layers are offices and service areas; and the bottom layers would be devoted to harvesting geothermal energy, according to the designers.

PopSci Choice Award: Moonscraper

Any discussion of the future of buildings wouldn’t be complete without at least mentioning the prospect of living somewhere other than Earth. The Moonscraper concept satisfies our high hopes, soaring above the lunar surface to reach precious sunlight. It would be situated at the rim of Shackleton Crater on the south pole of the moon, where, as we all now know, there is plenty of water. A nested system of vertical towers beneath the lunar surface would protect inhabitants from radiation, meteors and widely varying temperatures.