After this article was published, the Authors requested to change the title and abstract back to their original submission, since the removal of mention of the ngEHT project from the title and abstract during revisions does not accurately convey nor reflect the fact that this work was initiated and undertaken as part of the ngEHT project. The changes made following suggestions by a reviewer had not been approved by all authors/project director. Instead of “Fundamental physics opportunities with future ground-based mm/sub-mm VLBI arrays” the title should read “Fundamental physics opportunities with the next-generation Event Horizon Telescope”. The running title should be changed from “Fundamental physics opportunities with future ground…” to “Fundamental physics opportunities with the ngEHT”. Also, the current Abstract “The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems. Current ground-based very-long-baseline interferometry (VLBI) arrays like the EHT and proposed future extensions like the next-generation Event Horizon Telescope will greatly enhance the capabilities of black-hole imaging interferometry. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that future mm/sub-mm VLBI developments will enable” should be changed to “The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT) project, which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable”. The Original Article has been corrected.