Ground reality: India's megacities are slowly sinking - over 23,000 buildings may face severe damage

NEW DELHI: As India’s megacities expand skyward, the ground beneath them is quietly giving way — raising the question of whether the country’s next big urban crisis may be one that literally comes from below.

Over 2,400 buildings across Delhi, Mumbai and Chennai are already at high risk of structural damage due to land movement, while nearly 1.9 million people and more than 13 million buildings in five Indian megacities are exposed to ground subsidence (the gradual sinking or lowering of the Earth’s surface due to groundwater loss, soil compaction or underground excavation) rates exceeding 4 mm a year, a new study has found.

Published in Nature Sustainability and led by researchers from Virginia Tech and the United Nations University Institute for Water, Environment and Health, the study has warned that over 23,000 structures could face very high risk of failure within the next 50 years if current trends continue unchecked.

It used Sentinel-1 satellite radar data from 2015 to 2023 (varying by city) to map vertical land movement across Delhi, Mumbai, Chennai, Kolkata and Bengaluru. It found that 878 square km of land across these cities is subsiding, underscoring a growing urban hazard linked to groundwater depletion, rapid construction activity and climate variability.

This is the first large-scale analysis to quantify how differential land settlement — uneven ground sinking — could strain or fracture buildings across India’s urban landscape. The researchers observed that Delhi recorded the most severe and widespread sinking, with rates touching 51 mm per year in some pockets.

“The primary driver of land subsidence in Delhi is the compaction of alluvial deposits caused by extensive groundwater withdrawals,” the study noted. Chennai’s floodplain areas such as Valasaravakkam, Kodambakkam, Alandur and Tondiarpet showed sharp sinking due to groundwater extraction, while Mumbai exhibited slower rates overall, except in “economically disadvantaged neighbourhoods (for example, Dharavi) with high-density informal settlements”.

Though some areas in Delhi such as Dwarka have experienced uplift due to rainwater harvesting and restoration of water bodies, the overall pattern shows a worrying correlation between aquifer stress (pressure or depletion of underground water reserves) and ground movement (vertical shifting of the surface due to loss of soil stability).

The study noted that though land subsidence alone does not cause collapses, its “compounding effect and potential to cause long-term structural weakening are often ignored or overlooked when designing new infrastructure or planning maintenance”.

It estimated that 2,264 buildings in Delhi, 110 in Mumbai and 32 in Chennai already fall in the high-damage-risk category, while thousands more are in medium-risk zones.

The report highlighted that “a designation of high or very high risk of building damage does not imply imminent structural failure. Instead, it indicates that differential subsidence substantially increases vulnerability, potentially amplifying the impacts of other hazards such as flooding, storms or inadequate construction practices.” The study linked these findings to the broader challenge of rapid urbanisation — with over 83 million people living in India’s five megacities — and warned that groundwater depletion, unplanned growth and climate extremes are weakening urban foundations.

The authors called for a “holistic preventative approach” combining better building codes, stricter groundwater regulation and the integration of InSAR (Interferometric Synthetic Aperture Radar, a radar technology for detecting land deformation) into city planning. “Using this approach,” they concluded, “societies can better protect their infrastructure and ensure the safety and well-being of their inhabitants.”