Earthscrapers, tunnel towns and underwater restaurants – the trips of the future are literally groundbreaking .

Did you know that in 1900, there were just 1.6 billion people on our planet? During the 20th century, the population then exploded to more than six billion by 2000. Today, the population stands at 7.6 billion, with cities becoming more congested, polluted and overpopulated as economic migrants flock to urban areas. For some time, the most logical way to cram everyone in has been to build taller, narrower skyscrapers. But as real estate in prime areas becomes increasingly scarce, developers are having to become disruptors. For the traveller of tomorrow, journeys may be as much about “underworlds” as castles in the air.

In cities such as Toronto and Montreal, which suffer from particularly brutal winters, we can already see examples of how this is possible. The Path is a 30km network of pedestrian tunnels below Toronto’s pavements. Here you will find everything from cafés and supermarkets to subway stations and office entrances, so that you never have to go above ground. Bigger still, Montreal’s Réso has 1,700 boutiques and 200 restaurants, as well as cinemas, an ice rink, galleries, hotels and apartments. In London, Growing Underground is a space-age hydroponic farm, 33m below leafy Clapham, that produces edible herbs and salads without sunlight. Meanwhile, a limestone mine in Kansas City has been turned into SubTropolis, 5sq km of artificial caves that are being used as an industrial park and storage facility.

Inspired by their example, the island of Singapore (which spans a limited 278 square miles) is planning an Underground Science City campus for 4,200 scientists and researchers. Made up of 40 caverns going as deep as 30 storeys below, there will be space for data centres and research labs when it’s completed in the early 2020s. There are even hopes to take stadiums and libraries underground. At the same time, Bunker Arquitectura wants to delve into the bowels of Mexico City to install an Earthscraper that will take the form of an inverted pyramid that will sink 65 floors into the ground. Inside, it will have a light-filled atrium plunging through its core. By 2020, New York wants to inaugurate the Lowline, the world’s first solar-powered underground park, in the abandoned Williamsburg Bridge Trolley Terminal on the Lower East Side. Scroll down to find out why we might all soon be going underground on our next holiday.

In the US, you will find hotel suites carved out of the rock at the Grand Canyon Caverns in Arizona (this year it also opened a restaurant) and Kokopelli’s Cave in New Mexico. While over in Australia you can stay in the White Cliffs Underground Motel in the desert of New South Wales (set to reopen by summer 2018, after a refurbishment). But it’s the building going on in city centres that’s really causing a stir. In London, despite rejected blueprints and local councillors dismissing it as a property for “a bunch of troglodytes”, construction for an underground hotel is set to go ahead. It will take over a subterranean car park in Bloomsbury, and have 166 minimalist and, indeed, windowless rooms. The expectation is that prices will be lower, given you’ve got no view.

In Stockholm, the curious two-year-old Hotel With resides below its street-level sister establishment Urban Deli, which takes the form of an expansive food court, at Sveavägen 44. When shopping for an open sandwich, you’d never know the hotel was there, its 106 bedrooms sunk into windowless underground chambers connected by corridors with doors painted in primary colours. Interiors are far from dingy, though, with slick lighting, gleaming metro-tiled bathrooms, high-spec entertainment systems (rooms are completely soundproof), breezy ventilation systems and super-fast Wi-Fi. For the jet-lagged flyer who wants to sleep undisturbed, this could be the perfect hideaway.

We have the Victorians to thank for creating the first subways with the London Underground, but 21st-century innovators are taking networks to the next level. Billionaire entrepreneur Mr Elon Musk is working on a variety of sci-fi methods of travel. Mr Musk founded The Boring Company in 2016 with the intention of creating something anything but. He plans to cut new underground road networks beneath Los Angeles, taking the strain off its gridlocked streets. They won’t be tunnels you drive through, though. Instead, it will be like a metro system for cars, with vehicles whizzed from A to B on drive-on “skates” that shoot across the city at up to 150mph. (It will also have a pedestrian carriage.) Why hasn’t this been done before? The Boring Company says that tunnelling is extremely expensive, costing up to $1bn a mile, but Mr Musk hopes to bring the price down, just as he hopes to do with his space travel company, SpaceX.

Operating at far greater speeds will be Mr Musk’s Hyperloop One (now known as Virgin Hyperloop One after Sir Richard Branson recently invested undisclosed millions in the project), which will enable ultra-fast trains to travel at up to 670mph between cities. The Hyperloop will rely on tunnels as well, but instead of wheels and rails, carriages will be propelled through tubes using close to frictionless “magnetic levitation”. Tests are being conducted in the desert outside Las Vegas, and it is hoped that the first line will go live in 2021. So far there are 10 potential routes in five countries – the UK, US, Canada, India and Mexico. In the UK there is talk of a London to Edinburgh journey that would take just 41 minutes (normally more than four hours by train) and, in the US, Miami to Orlando, which would be 27 minutes (typically three-and-a-half hours by car).

Mr Bill Gates is rumoured to have several subsurface houses already and in 2016, after the election of Mr Donald Trump and his consequent ding dongs with Mr Kim Jong-un of North Korea, Texan bunker builder Rising S Company has reported a 700 per cent increase in demand for its impenetrable bunkers. Many companies, such as Vivos, are renovating existing bunkers rather than building them from scratch. In South Dakota, for example, Vivos is transforming a former WWII munitions depot into a sprawling complex called Vivos xPoint, for 5,000 people. For between $25,000 and $200,000, you can kit out your underground apartment with Persian rugs, dark polished floors and brown leather sofas. And to make sure you don’t get bunker fever, there will be a restaurant and bar, a theatre, a hot tub and spa, a gym, hydroponic gardens, shooting ranges and barbecue areas.

Meanwhile, enterpreneur Mr Jakub Zamrazil is turning an ex-nuclear bunker in the Czech Republic into The Oppidum, one of the biggest residential bunkers in the world at 7,200sq m. Among its many facilities, it will have seven apartments, a swimming pool, a cinema, a wine vault and simulated daylight so you don’t wilt. The good news for fans of the sharing economy is that, unless North Korea decides to drop the bomb, it is likely these will start appearing for rental online. (Last year, Subterra Castle in Kansas was listed on Airbnb for £106 a night.)

The most southerly point of Norway’s coast, Lindesnes, may not be suffering from overcrowding, but remote locations such as these are already becoming desirable wilderness escapes for the burnt-out and tech-weary. As back-to-nature travel booms, we will start seeing an increase in imaginatively designed developments that are integrated into forests, glaciers, mountains, deserts, jungles and even under the sea. And in Lindesnes, Norwegian design firm Snøhetta, which recently designed one of the cabins at Sweden’s Instagram-friendly Treehotel, has revealed plans to build Europe’s first underwater restaurant.

The monolithic structure will look like an abandoned piece of concrete that has slid down from the beach, the dining room facing the ocean floor via a huge acrylic window. Protected by 1m-thick acrylic windows, up to 100 guests will be able to stop first for a glass of champagne at the bar, before feasting on lobster while gazing out at the ever-changing scenery. Under, as the restaurant will be called, will double up as a research laboratory for marine life, and is expected to open in spring 2019. You will also now find underwater restaurants in the Maldives – the Conrad has Ithaa and the Hurawalhi has 5.8 Undersea, with a submerged glass tunnel with views of a coral reef.