Yemen Monitor/Investigations Unit/Exclusive:
The US launched airstrikes on the Yemeni mainland, claiming to have targeted underground sites belonging to the Houthi armed group. This is the first time the Pentagon has used bombers specializing in targeting underground fortifications since the start of attacks in Yemen last January.
Over the years of war, the Houthis have developed their underground bases to avoid attacks led by Saudi Arabia in support of the internationally recognized government, a tactic the armed group has learned during the six wars with the government (2004-2009).
After the announcement of the truce in April 2022, the Houthis expanded their construction and establishment of underground bases, according to multiple sources who spoke to Yemen Monitor. Most of the group’s tunnels have been transformed into the construction of underground military cities, a new phase of protection for a long war that the Houthis expect to face in the future. This came after their participation in the Red Sea attacks, which they say is in support of the Palestinians who are facing a brutal war and genocide in the Gaza Strip by the Israeli occupation.
According to the sources, the intelligence breach that occurred to Iran and Hezbollah represents the biggest concern for the Houthi leadership. Although targeting underground camps and factories is a less urgent concern at the moment, many of the group’s leaders believe it is necessary to prioritize it because the Houthis’ regional involvement is sudden and US and Israeli intelligence had not conducted widespread infiltration within the main ranks of the group.
So what is the nature of these underground military cities, the Houthis’ hidden world? Yemen Monitor spoke to more than 12 informed sources about the details to answer this question.
Nature, Types, and Scale of Houthi Underground Facilities
The exact size and depth of these underground cities remain unknown. However, the US use of stealth bombers (B-2 Spirit) on October 17, 2024, to target underground weapons storage has reignited discussions about these facilities.
Three heavy equipment dealers in Sana’a (capital), Hajjah (north), and Hodeidah (west) told Yemen Monitor that the Houthis have been regular customers for their machinery since 2018, with a surge in demand for machinery, either through rental or purchase, since 2022.
They indicated that the rental process is carried out through supervisors and officials affiliated with the group, including at least one who used to be a contractor for the Ministry of Defense and is associated with “Ahmed Hamid” (Abu Mahfouz), a powerful figure within the Houthis and director of the office of Mehdi al-Mashat, the head of the Supreme Political Council (a body equivalent to the presidency).
In fact, Yemen has had underground facilities for decades. Former Yemeni President Ali Abdullah Saleh built a number of underground facilities for ballistic missiles and important weapons in and around Sana’a.
Two Houthi sources with knowledge of the details told Yemen Monitor that the group has renovated and expanded existing underground facilities built during the era of Ali Abdullah Saleh and seized by the Houthis after killing him in late 2017 following his attempt to switch his allegiance from the armed group to the Saudi-led Arab coalition.
The sources indicate that the Houthis have since built a large number of underground warehouses and factories for various purposes: assembling and manufacturing ballistic missiles and drones, explosives manufacturing plants, assembly and manufacturing plants for unmanned boats and sea mines, command and control, for war, for conventional weapons storage, and for advanced weapons storage.
Iran faces accusations of supplying the Houthis with weapons technology, including 2000km-range missiles, hypersonic missiles, and drones, some of which were used last year to attack Israel and ships in the Red Sea. The Iranian regime denies arming the Houthis, despite the discovery of weapons manufactured in Tehran on the battlefield and in sea shipments headed to Yemen, despite a UN arms embargo.
Depth: The Cornerstone of Protection and Secrecy
Given the underground nature of these facilities, their depth is the cornerstone of protection and secrecy. In April 2017, intelligence reports revealed that the Houthis were building their factories 50 feet underground.
These reports indicated that experts from the Lebanese Hezbollah, the Iranian Revolutionary Guard, and experts from the Iranian Aerospace Force were in Yemen to build these facilities.
Sources from “Yemen Monitor” indicate that what was built at that time were simple factories. As the Houthis expanded their armament capabilities, developing their own weapons and increasing their destructive power, and conducting tests to assess their success, they needed to expand and construct new facilities.
A source stated that the group’s command and control facilities during the war are spread across multiple areas, including the capital Sana’a, and are built 7-10 stories underground. Large commercial warehouses are used as a cover for their existence.
Protection
For underground military factories and workshops, the Houthis are building thick concrete-lined ceilings to provide protection from airstrikes, as indicated in a report published by “Fleet Germany” on April 27, 2017.
Satellite images released by a US research center last April show the Houthis using rugged terrain to build weapons and explosives storage facilities and weapons factories. The images show large craters created by the Houthis in Yemeni mountains last year that can be entered by armored vehicles.
Yemen Monitor sources said that there are advanced ventilation systems in these facilities to prevent suffocation, including alarms and other safety measures.
Although there is no evidence of safety and prevention policies in such facilities, one source noted that the Houthis have improved safety measures with the occurrence of numerous accidents in these facilities over the past decade. “We learn from experience over time.”
Secrecy
Secrecy about the locations of these facilities serves as an additional layer of protection. You can’t simply navigate to them using Google Maps; these facilities are not tourist attractions for group members, and even those connected to the facilities are subject to strict access policies.
According to a Houthi official, these facilities were designed in terms of location and cover to be secret and difficult to discover, reducing the likelihood of attacks.
These facilities are located in the bellies of rugged and towering mountains in northern Yemen. To reach them, the Houthis have constructed secondary roads that were carved out during years of war and are not connected to known roads.
One of the three heavy equipment businessmen who spoke to Yemen Monitor said that they helped the Houthis build some roads in the governorates, and that civilians are prohibited from using them.
The Houthis rely on their recruits and loyalists to help pave and carve these roads, according to five of the group’s recruits who spoke at a qat session in Amran Governorate last May in the presence of a Yemen Monitor source.
The recruits indicated that each team works on a specific part of the road and then leaves, with new teams coming in without knowing the final end of the road and its purpose.
These facilities are directly managed by the office of the Houthi leader, with multiple groups responsible for protection, secrecy, and transporting food, experts, and supplies they need, most of which come from various departments in the Ministry of Defense, primarily the Military Intelligence and Reconnaissance Directorate headed by Abdullah Yahya al-Hakim (Abu Ali).
New Variables
There is no doubt that the Houthis are learning from their past experiences and from other armed movements in the region. The tunnels in Gaza and their ability to withstand and survive Israeli attacks for a year have inspired the Yemeni movement.
The same applies to the nature of the targeted assassinations that Hezbollah leaders have been subjected to in Lebanon; the US targeting using B-2 Spirit bombers indicates the extent to which targeted US strikes against the group can go, and they are seeking solutions to the weaknesses they have discovered with the new variables.
Houthi leaders expect a targeted war against the group in the near future, either from their opponents in the internationally recognized government or through an international coalition that surpasses the airstrikes carried out by the US and Britain since January. They say they are preparing for it.
