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When will the Challenger Deep cease to be the deepest ocean trench? And how will this occur?

• The , whose deepest point below sea level is Challenger Deep (10,898 metres), is the result of the planet’s largest tectonic plate, the Pacific plate, slipping under its neighbour the Mariana plate. This process of one plate sliding under another is called subduction.

Ocean trenches are formed over millions of years, and the simple answer to the above question is: nobody knows. Challenger Deep may always be the deepest point beneath sea level because of the relative masses of the plates that form it.

Interestingly, Challenger Deep is not the point on the surface that is nearest to the centre of the planet. Because Earth is flattened at the poles, that point on the seabed is likely to be in the Arctic Ocean.

Tony Holkham, Boncath, Pembrokeshire, UK

• Exactly when the forces that created Challenger Deep will moderate we cannot say. What is clear is that as soon as they stop, the hole will begin to fill with biological detritus, chemical concretions and the products resulting from erosion of the surrounding topography. Once that begins, the process should be pretty quick and it might be difficult to locate the trench 1 million years or so after its generating forces have abated. Structures such as this tend to be geologically short-lived. By way of comparison, a 1-million-year-old lake is a pretty old one – it will usually have silted up, dried up or eroded long before that.

The reason the African Great Lakes are still there is that the Great Rift Valley is still rifting – they are, in fact, an ocean in embryo. It is a similar story with oceanic deeps; remove the forces that cause them, and they pass away like dimples in foam.

Jon Richfield, Somerset West, South Africa