The first ever natural hot spring has been found in the heart of Tokyo’s Otemachi financial district by Japan’s biggest developer by market value, Mitsubishi Estate, after digging 1.5km (0.9 miles) deep.

Mitsubishi Estate was told two years ago that the likelihood of thermal waters in the region was high, so it developed a plan for a natural spa in the most expensive business district in the country – where executives have a love for hot spring bathing, known as "onsen".

The digging began in April and the thermal water was found – pumping at rate of 240 litres per minute – in June. The water will be 36.5ºC and contains high levels of sodium chloride. Thermal waters are thought to help fatigue, poor blood circulation, depression and skin dryness. A spokesperson for Mitsubishi Estate told Bloomberg that the project is set to be completed in 2016 – in time for the 2020 Olympics.

The development is expected to cost more than 100 billion JPY (US$983m, €726m, £534m) according to news site ajw.asahi.com.

The project includes an 18-storey hotel building which will be run by luxury hotel operator Hoshino Resort Co, and next to it will be a 31-storey office tower which may also include fitness and spa facilities of its own, according to Mitsubishi Estate – quoted by Bloomberg.

The hotel is expected to house a large rooftop pool and the baths in each of its guestrooms will be fed from the hot spring.

The adjacent office building will also be equipped with shower rooms and baths which will be made available not only to people working in the building, but also to those working or jogging in the neighbourhood.

“Foreigners who visit Tokyo have told us that they want to take authentic onsen baths,” Mitsubishi Estate president Hirotaka Sugiyama told ajw.asahi.com. “We want to make the atmosphere of this hotel different from those in surrounding areas.”