Maybe it shouldn’t be that surprising that Amazon has patented a system for delivering goods via a dedicated network of underground tunnels. After all, the Seattle-based company is looking into virtually every other mode of transportation.

But the idea seem ambitious, even for America’s largest online retailer.

Amazon has experimented with delivery services that make use of autonomous drones, bicycle couriers and branded fleets of airplanes and trucks. There’s talk of self-driving trucks, flying warehouses and a system that would let drones hitchhike on trucks and buses.

Even the patent for Amazon’s Treasure Truck leaves the door open for Treasure Boats as well.

Amazon’s patent application for a dedicated network delivery system, above or below ground, was filed almost three years ago. The patent was finally issued and published a little more than a month ago.

The 33-page application lays out the specifications for a network that could transport packages via conveyor belts or rails, or even through pneumatic tubes. (Hyperloop fans, take note.)

The system could connect rail stations, airports, fulfillment centers, locker storage site and, of course, customers. In the application, Amazon inventor Xiaoshan Cai says such a system “may avoid congestion experienced by traditional transportation networks.”

There’s no indication that Amazon is actually planning to build such a network anytime soon, but other companies are thinking about it.

For instance, a British company called Mole Solutions has proposed building a network of pipelines for freight delivery. It’s even set up a 344-foot-long (105-meter-long) demonstrator track to test the technology.

And then there’s the Hyperloop concept, laid out three and a half years ago by Elon Musk, the billionaire CEO of SpaceX and Tesla. That concept calls for building a network of tubes that magnetically levitating pods can zoom through at near-supersonic speeds.

A couple of ventures, including Hyperloop One, are trying to commercialize the idea for freight shipments as well as passenger transport.

Hyperloop networks are typically envisioned as above-ground tube systems, but last month, Musk said he was seriously thinking about getting into the tunnel-boring business. He’s even added tunnels to his Twitter bio (along with Tesla, SpaceX and OpenAI).

Anyone familiar with the twists and turns of Seattle’s tunneling tale knows that boring a hole in the ground is not as easy as some might think. But is it possible that Musk and Amazon’s engineers are on to something? This may be a subject worth digging into after all.