A groundbreaking scientific expedition was just completed off the west coast of Hawaiʻi Island in search of something unexpected: fresh water beneath the ocean floor. Researchers from the University of Hawaiʻi at Mānoa and Scripps Institution of Oceanography teamed up for a two-week offshore imaging survey in July 2025, through a contract with the Natural Energy Laboratory of Hawaiʻi Authority (NELHA) and funding from the Hawaiʻi State Legislature.
The goal was to confirm the existence of a potentially massive underground reservoir of fresh or brackish water hidden beneath the seafloor—an idea that challenges conventional understanding of island hydrology.
“If proven, this deep water could explain long-standing mysteries about Hawaiʻi Island’s water cycle—namely, why observed coastal discharge doesn’t match estimated groundwater recharge,” said Peter Kannberg, associate researcher at the Hawaiʻi Institute of Geophysics and Planetology (HIGP) in the UH Mānoa School of Ocean and Earth Science and Technology, who led the survey. “In simple terms, a lot of water is missing from current models.”
The roots of this investigation trace back to 2018, when scientists first detected anomalies suggesting a deep, confined aquifer where none should exist. Their theory? Rainwater may be funneled underground trapped beneath layers of volcanic basalt and ash, extending far beneath the basal freshwater lens and even miles offshore.
“If confirmed, this hidden aquifer could reshape our understanding of island hydrology and inform future water resource planning—offering a potential new source of fresh water in a region increasingly affected by drought and climate change,” said Alex Leonard, senior project manager for NELHA.
“If a significant fraction of freshwater is escaping through these deep aquifers, then we need to re-calculate how much water can can be withdrawn from the nearshore basal lens for human consumption and how much needs to remain in these shoreline aquifers to provide nutrients to the reefs and nearshore marine environment,” added Don Thomas, HIGP faculty who has spent decades studying water and subsurface features across the Hawaiian Islands.
High-tech imaging survey takes to the sea
The 2025 survey used advanced electromagnetic imaging techniques. Instruments were deployed on the seafloor to listen for electromagnetic echoes, while a 150-foot-long towed sensor emitted a low-power signal that is recorded on a 3,300-foot-long receiver array. The study spanned two areas: north of Kiholo Bay to Honōkohau Harbor and the coastline south of Kailua Bay to Kealakekua.
The team will now begin processing data from the surveys to determine whether this reservoir is present and better understand how much water may be in this hidden offshore aquifer.
“We are now applying state-of-the-art technologies to better characterize the complexity of island hydrology—advancing knowledge that could fundamentally improve how we manage and sustain freshwater resources across volcanic islands both here and abroad,” said Amir Haroon, faculty member at HIGP, who studies water resources on Oʻahu and Maui.
More information
For more on the project and answers to frequently asked questions, visit NELHA.
