https://www.bbc.com/future/article/20190503-the-deepest-hole-we-have-ever-dug

The Soviets’ Kola Superdeep borehole isn’t alone. During the Cold War, there was a race by the superpowers to drill as deep as possible into the Earth’s crust – and even to reach the mantle of the planet itself.

Now the Japanese want to have a go. “The ultimate goal of the new project is to get actual living samples of the mantle as it exists right now,” says Sean Toczko, programme manager for the Japan Agency for Marine-Earth Science.

“When the Russians drilled they claimed they had found free water – and that was simply not believed by most scientists. There used to be common understanding among Western scientists that the crust was so dense 5km down that water could not permeate through it.”

(That argument fails to convince mollwollfumble, water is ingested at plate boundaries, and there’s also probably a lot of primordial water, from the formation of the Earth, trapped down there).

When in 1961 Project Mohole began to drill into the seabed, deep-sea drilling for oil and gas was still far off. No one had yet invented now essential technologies such as dynamic positioning, which allows a drill ship to stay in its position over the well.The hole drifted 200 m off vertical. Two years before Neil Armstrong walked on the moon, US Congress cancelled the funding for Project Mohole when costs began to spiral out of control

One of the biggest challenges engineers faced was the need to drill a hole that is as vertical as possible. The solution the Germans came up with is now a standard technology in the oil and gas fields of the world. “What was clear for the experience of the Russians was that you have to drill as vertical as possible because otherwise you increase torque on the drills and kinks in the hole,” says Uli Harms. “The solution was to develop vertical drilling systems. These are now an industry standard.

The German borehole (KTB) has been spared the fate of the others. The huge drill rig is still there – and a tourist attraction today – but today the crane just lowers instruments for measurement. The site has become in effect an observatory of the planet – or even an art gallery.

Today, the Japanese “M2M-MoHole to Mantle” is one of the most important projects of the International Ocean Discovery Program (IODP). As with the original Project Mohole, the scientists are planning to drill through the seabed where the crust is only about 6km (3.75 miles) deep. The goal of the $1bn ultradeep drilling project is to recover the in-situ mantle rocks for the first time in the human history.