Building an amphibious car has been one of the more vexing challenges for transportation engineers over the years. Some are fast on land and slow in the water. Vice versa on others. It's hard to build a vehicle capable of handling both modes.

Over the past six years, designer Marc Witt has built a prototype that can reach speeds of 125 mph on land and 60 mph in the water. The Sea Lion, is in competition with 25 other models to claim the title of the world's fastest vehicle capable of both operations.

Now, the Sea Lion can be yours.

For the price of $259,900, Witt is selling the Sea Lion on an auto enthusiast website called fantasyjunction.com. (The site is safe for work; we checked). The vehicle features a Mazda Rotary 13b engine and four-speed transmission.

"I am willing to continue working with the buyer on all future modifications," Witt writes. "I will remain available as a consultant, engineer, machinist and psychiatrist for whoever decides to venture further into the amphibious record books."

Competition between Witt's prototype and the 25 other contenders for the title of World's Fastest Amphibious Car is somewhat unofficial – there are no official rules to the contest and each year, one builder proclaims their performance better than the rest. Guinness Book Of World Records, however, will verify a true winner should a conflict develop.

Witt says the current Sea Lion is limited by horsepower in the water, and the next owner should consider upgrading the engine to keep pace with competitors.

"The next chapter of this project is to provide an ultimate engine and begin speed trials," he writes.