What just happened? If you’re the second-richest person in the world with an eco-friendly outlook, what toy would you buy? For Bill Gates and his $100 billion fortune, it’s a hydrogen-powered superyacht. The $644 million vessel comes with five decks, an infinity pool, gym, helipad and spa.

Update: Despite the Sunday Telegraph’s claims, it seems Bill Gates isn’t buying the Aqua. “The Hydrogen concept Aqua is not linked to Mr Gates (or his representatives) in any form or matter,” Sinot Yacht Architecture & Design said in a statement. “Sinot has no business relationship with Mr Gates.” The company added that the yacht is “a concept under development and has not been sold to Mr Gates.”

The original story follows below:

Gates has reportedly ordered Aqua, a 376-foot long concept design by Sinot Yacht Architecture & Design and Lateral Naval Architects superyacht design and construction. Unveiled at the Monaco yacht show, it has room for 14 guests and 31 crew members and boasts a 3,750-mile range with a top speed of 17 knots (19.6 mph).

The yacht's marquee feature is its liquid hydrogen and fuel cell system. Below decks are two 28-tonne vacuum-sealed tanks that are cooled to -423F and filled with liquid hydrogen. This is converted into electrical energy by Proton Exchange Membrane (PEM) fuel cells, leaving only water as a by-product. The system provides power for the two one-megawatt motors and propellors, along with other systems.

While the yacht runs on eco-friendly liquid hydrogen, the dearth of hydrogen refueling stations means it also has a back-up diesel engine.

Other luxury features include floor-to-ceiling windows, a massage room, yoga area, and a huge circular staircase.

“For the development of Aqua we took inspiration from the lifestyle of a discerning, forward-looking owner, the fluid versatility of water and cutting-edge technology, to combine this in a superyacht with truly innovative features,” said designer Sander Sinot.

Gates has previously shown his support for zero-emissions technology though his investment in clean energy company Heliogen, which uses advanced computer vision software to create concentrated solar energy that could replace fossil fuels in key industrial processes.

Aqua isn’t expected to be ready for the seas until at least 2024, so Gates will likely be renting his superyachts for a few more years.