An aircraft traveling approximately 460 mph at an altitude of 40,000 feet could someday produce 3-D images of structures hidden deep beneath Earth’s surface, allowing operators to investigate underground targets without flying directly overhead.
The Defense Advanced Research Projects Agency (DARPA) wants to develop precisely that capability. However, several unusually specific requirements in its recently published solicitation raise questions about whether the Pentagon’s interest goes beyond geological exploration.
DARPA is seeking a self-contained radio-frequency sensing system that can rapidly produce 3-D images of underground features from high-altitude aircraft. The technology must operate at about 40,000 feet and 400 knots and function as a modular payload that can be installed on existing fixed-wing aircraft without extensive permanent modifications.
The agency also wants the technology developed fairly quickly.
Rather than soliciting early-stage concepts, DARPA requires prospective contractors to demonstrate that they have already developed and flight-tested comparable subsurface sensing systems. Its proposed development schedule calls for preliminary high-altitude flight demonstrations within 15 months and completed three-dimensional reconstructions by month 18.
Although DARPA identifies critical-mineral exploration as a primary purpose, the combination of rapid development, precise operating requirements, and modular aircraft integration suggests it could have additional military intelligence, surveillance, and reconnaissance (ISR) applications.
Notably, DARPA’s solicitation specifically calls for “rapid, stand-off 3D subsurface tomographic imaging to detect subterranean features.” The emphasis on “stand-off” operation raises the possibility of applications involving intelligence collection from outside contested or denied airspace. However, DARPA has not publicly identified those missions as program objectives.
When contacted by The Debrief, DARPA’s public affairs office declined to elaborate on the program, stating that it was “not able to comment” beyond the information already provided in the initial request for information (RFI).
WHY DARPA WANTS TO SEE UNDERGROUND FROM EIGHT MILES UP
DARPA’s solicitation, titled Novel Radio Frequency Sensing Technologies, describes a system that can detect and map underground features using high-powered radio-frequency technology.
The agency’s stated objective is to identify additional domestic sources of critical minerals, which remain essential to advanced military technologies and defense manufacturing.
For commercial applications, DARPA describes an ambitious vision resembling an “MRI for the Earth.” Such a system could potentially help identify critical minerals and rare earth deposits hidden under hundreds of feet of geological material.
However, the requirements extend well beyond those of many conventional airborne geological surveys.
Both scientific and military aircraft already deploy specialized sensing technologies to investigate what lies beneath Earth’s surface. Existing systems support applications ranging from geological mapping and mineral exploration to detecting underground structures.
Still, conventional airborne electromagnetic sensing typically operates much closer to the ground. For comparison, a U.S. Geological Survey operation over California’s western Mojave Desert in November 2024 used a helicopter carrying electromagnetic and magnetic instruments about 115 feet above the terrain.
The survey collected measurements along roughly 2,020 miles of flight lines to support geological mapping and critical mineral exploration.
DARPA’s proposed operating altitude is nearly 350 times higher than that particular survey. Although conventional airborne electromagnetic surveying and DARPA’s proposed radio-frequency sensing system are not necessarily equivalent technologies, the comparison illustrates the substantial difference in operating conditions.
It also raises an important question: Why does DARPA specifically require a system capable of operating at the altitudes and speeds associated with high-performance fixed-wing aircraft?
One potential answer could involve the ability to survey larger geographical areas more quickly. However, high-altitude operations and stand-off sensing would offer obvious advantages for military intelligence collection, possibly allowing aircraft to investigate underground targets from greater distances.
Whether the proposed technology could detect deeply buried military infrastructure remains unknown.
A SELF-CONTAINED SYSTEM DESIGNED FOR EXISTING AIRCRAFT
One of the solicitation’s most significant requirements concerns how the technology would be deployed.
DARPA is not simply asking contractors to develop a powerful underground radar. It wants a complete, self-contained sensing payload that can be integrated into existing high-altitude fixed-wing aircraft without expensive or permanent modifications.
“To facilitate rapid, modular deployment across diverse airframes without requiring permanent, costly aircraft modifications, the entire system must be designed as a fully self-contained payload,” DARPA’s solicitation reads. “It must utilize independent, internal power generation and an edge computational architecture.”
These requirements introduce considerable engineering challenges.
A PHYSICS PROBLEM THAT COULD REQUIRE 256 TIMES MORE POWER
According to DARPA, increasing the operating altitude from 10,000 to 40,000 feet could theoretically require 256 times more radiated power to maintain equivalent imaging performance under the radar-scaling assumptions described in the solicitation.
For small radar targets with particular characteristics, the necessary transmitter power increases approximately with the fourth power of distance. Quadrupling the distance can therefore increase the required power by 256-fold.
Those calculations do not apply universally to every electromagnetic sensing method, but they illustrate the engineering challenge DARPA wants contractors to address. The proposed system must incorporate a reliable kilowatt-class radio-frequency amplifier and an antenna capable of resisting aerodynamic forces encountered at approximately 460 mph.
It must also deal with heat from sustained high-power transmissions, prevent interference with the host aircraft’s navigation and mission systems, and process incoming measurements using onboard computing hardware.
Crucially, the payload must include its own power generation and computing capabilities. These requirements could allow a successful system transfer between compatible aircraft without major modifications, likely reducing the time and expense of deploying new sensing capabilities.
For military applications, such flexibility could be remarkably valuable. Although DARPA has not revealed which aircraft might carry the proposed system, its precise operating requirements are noteworthy.
The specified altitude of 40,000 feet and speed of 400 knots overlap the performance envelopes of existing U.S. ISR aircraft, including the RC-135 Rivet Joint and P-8 Poseidon.
While these similarities do not establish which platform DARPA has in mind, they suggest the agency’s requirements align with the capabilities of at least some existing military reconnaissance aircraft.
DARPA WANTS FLIGHT-TESTED TECHNOLOGY ON AN ACCELERATED SCHEDULE
The program’s development timeline also suggests DARPA’s interest isn’t limited to preliminary research.
Published in early September, the opportunity is structured as a Direct-to-Phase-II Small Business Innovation Research solicitation. Instead of funding an initial phase to determine whether a proposed technology is feasible, DARPA is seeking contractors that can document relevant capabilities they have already developed and tested.
Applicants must deliver empirical flight data from an airborne subsurface radar or comparable sensing system, including representative 3-D reconstructions, signal-to-noise calculations, and evidence of successful underground mineral detection.
Computer simulations alone will not satisfy these requirements; successful applicants would then face an accelerated hardware development and flight-testing schedule. Within six months, contractors would be expected to demonstrate progress in manufacturing and testing major components, including their power amplifiers and antenna assemblies.
By month nine, the program calls for progress on integrated systems testing and onboard 3-D reconstruction capabilities. However, the most consequential milestone arrives at month 15, when DARPA expects preliminary data from demonstration flights carried out using fixed-wing aircraft operating at approximately 40,000 feet and 400 knots.
By month 18, contractors would be expected to deliver reconstructed 3-D subsurface images of test sites and a detailed assessment of system performance. An optional six-month extension would permit further testing and improvements.
These milestones are proposed contractual requirements, not evidence that a successful high-altitude system already exists.
Nevertheless, requiring proven underlying technology, specifying an existing-aircraft payload, and establishing high-altitude flight demonstrations within 15 months together indicate that DARPA is pursuing an accelerated development effort rather than an open-ended scientific investigation.
What remains unclear is whether DARPA has a particular operational mission in mind. The solicitation identifies critical-mineral exploration as a “national security imperative”. It also spotlights the technology’s potential to accelerate the discovery of mineral reserves and rare earth elements for commercial and civilian applications.
COULD THE TECHNOLOGY HAVE APPLICATIONS AGAINST UNDERGROUND MILITARY FACILITIES?
The potential military intelligence applications become particularly relevant when considered alongside the continuing conflict between the U.S. and Iran.
Iran has invested heavily in underground nuclear infrastructure, including facilities at Fordow and Natanz, while questions surrounding its nuclear activities and access to affected facilities remain unresolved.
The International Atomic Energy Agency (IAEA) has documented significant gaps in its ability to verify Iran’s nuclear activities following military strikes against several nuclear facilities and subsequent restrictions on inspections. Those uncertainties have increased the relevance of understanding the condition and activities of Iran’s remaining nuclear infrastructure.
A technology capable of producing high-resolution three-dimensional images of underground structures from stand-off distances could, in principle, offer intelligence agencies another way to investigate subterranean facilities.
And unlike conventional overhead imagery, which primarily reveals surface activity and visible infrastructure, subsurface tomography could potentially provide information about features covered beneath the ground.
However, substantial technical uncertainties endure. The ability of radio-frequency signals to penetrate the ground relies on factors including operating frequency, subsurface conductivity, geological composition, and target characteristics.
Different materials can absorb, scatter, or otherwise alter electromagnetic signals, complicating attempts to identify deeply buried structures.
Consequently, detecting underground geologic features does not establish that the proposed system could identify hardened military installations or produce useful images of facilities entombed beneath substantial layers of rock.
Nor does the solicitation establish that DARPA’s program is connected to any concerns with underground facilities in Iran, or any other potential near-peer adversaries like China or Russia.
Nevertheless, the timing of the announcement, the requirement for stand-off imaging, the emphasis on compatibility with existing aircraft, and the accelerated development schedule make the potential military applications an important unanswered question.
Ultimately, the publicly available solicitation does not identify a prospective aircraft, disclose an operational deployment plan, or specify a foreign intelligence target.
What it does establish is that DARPA wants a rapidly developed, modular sensing system that can produce 3-D underground imagery while operating at altitudes and speeds far beyond those used in many conventional geological surveys.
If contractors meet the agency’s proposed milestones, the resulting demonstrations could help determine whether high-altitude subsurface tomography can deliver the performance necessary for DARPA’s stated mineral-exploration objectives, and potentially establish the technical foundation for additional military sensing applications.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com