Miami ‘Underline’ Delivers Fun and Function Beneath Elevated Metro Tracks

From shorelines to streetways and from rooftops to rivers, an ever-expanding catalogue of nature-based stormwater management projects is demonstrating that no setting is too constrictive for green infrastructure. In Miami, a project 10 years in the making is adding another unconventional site to the list: a thin, 16-km-long (10-mi-long) strip of land beneath the city’s elevated mass transit system.

Miami’s The Underline project, like all green infrastructure, aims to achieve multiple goals within its narrow footprint. Adding new walking and biking paths, it endeavors to expand pedestrian connectivity for one of the world’s most traffic-choked cities. An array of amenities such as butterfly gardens, dog parks, and outdoor exercise equipment gives locals new ways to engage with their city. And an expansive system of bioswales, stormwater ponds, rain gardens, and other natural installations improves management of both the quantity and quality of runoff in a notoriously storm-prone urban center.

The final and most extensive phase of The Underline’s construction now is underway, with the existing stretches already attracting more than 2 million visitors each year. Patrice Gillespie Smith, President and Chief Operating Officer of Friends of The Underline (Coral Gables, Florida) — a nonprofit group working with Miami-Dade County to design and deliver the project —anticipates a ribbon-cutting in 2026. When complete, she hopes The Underline will prompt developers around the world to view overlooked urban spaces in a fresh, green light.

“This is something every city should be looking at — how to repurpose underutilized land — especially as all of us are seeing more and more development in our urban areas,” Smith said. “We all need a connection to nature, regardless of where we live.”

Addressing Safety and Stormwater Management

Recognizing the inherent value of the land beneath the city’s Metrorail system, Miami-Dade County established the M-Path ­— a paved corridor targeting cyclists — in 1983. However, by the mid-2010s, the M-Path had fallen into disrepair. Users reported poor pavement quality as well as rampant obstacles such as tree roots that make traversal risky.

To make matters worse, Smith said, the M-Path’s location beneath a railway has exposed it to soil contamination, mainly from arsenic and other heavy metals. This poor soil functionality creates, among other issues, problems for stormwater management. South Florida’s characteristic heavy downpours fall in thick columns from above the tracks, and lacking proper infiltration, much of this runoff creates flood potential while also carrying contaminants into the ecologically sensitive Miami River and Biscayne Bay.

The M-Path tells a broader tale of Miami’s lack of safe pathways for nondrivers. According to Transit Alliance Miami, pedestrian and cyclist fatalities in Miami-Dade County are nearly twice the national average when adjusted for population.

“We’re a newer city, and we have very wide roads in a lot of cases,” Smith said. “In older and more historic cities, a lot of those roads are narrow. It’s harder for cars to get up to a speed that will kill a cyclist if a collision happens.”

In fact, it was a 2013 cycling accident — incurred by Friends of The Underline founder Meg Daly — that inspired The Underline project. After breaking both her arms, Daly would walk along the M-Path to get to physical therapy, noticing the space’s unrealized potential. After she healed, she began engaging the county, urban designers, attorneys, cyclists, and all manner of volunteers to upgrade and modernize the M-Path into a multifunctional artery, supporting both nonmotorists as well as the environment. A decade later, the momentum behind Daly’s grassroots movement has not slowed.  

Strategic Site Seeding

The power of The Underline is in its strategic choice and use of native plants to support stormwater management, combat urban heat-island effects, provide new amenities, and improve pedestrian connectivity. Built on county-owned land connecting eight elevated Metrorail stations, the park’s various green infrastructure elements will feature more than 500,000 native plants and trees when completed, Smith said.

The Underline’s unique setting means not just any native plants will flourish there, however. With the railway tracks 6–12 m (20–50 ft) overhead, the amount of sun available to the plants lying beneath varies throughout each day. Friends of The Underline are working alongside horticulturists from the Fairchild Tropical Botanic Garden (Coral Gables, Florida) and the University of Florida Institute of Food and Agricultural Services (Gainesville) to choose species as well as specific planting sites.

The Underline’s first two phases have provided ample opportunities for trial and error, Smith said. As the partnership embarks on the final, 11.9-km (7.4-mi) phase, the project team has assembled a growing list of reliable species — such as water hyssop, buttonbush, and swamp milkweed — that are likely to thrive within The Underline’s bioswales and rain gardens.

“We have 180 different tree species just in our 2.6 miles [4 km] alone that are currently open, and we’re able to see which species thrives in which environment,” Smith said. “Some are more shade-tolerant while others need more direct sun, and so we’re able to adjust our plantings based on that evaluation.”

Perhaps the most innovative element of The Underline’s green infrastructure collection will be its system of 24 pocket forests — an approach to urban forestry growing in prominence worldwide. Pioneered by Japanese ecologist Akira Miyawaki, pocket forests are notable for their variety and density, both of which enable them to grow as much as 10 times faster than traditional, monoculture forests. They occupy parcels as compact as 3 m² (32 ft²) in area and feature diverse plantings that compete aggressively for limited resources, ensuring only the strongest seedlings propagate and thereby maximizing their ecological benefits within their small footprint. Consisting of nearly 24,000 individual seedlings, the pocket forests within The Underline are expected to provide “the cooling equivalent of five air-conditioning units working for 274 years with minimal maintenance,” Smith said.

Crucially, every piece of The Underline must contend with the site’s contaminated soil. With each new stretch of development, Underline crews are digging up this soil where practical and either replacing it or buffering it with at least 0.3 m (1 ft) of fresh soil, restoring infiltration capacity and keeping the space healthy for both plants and people. And because the design calls for only native plants, requirements for irrigation and nitrate-heavy fertilizers are virtually nonexistent.

“Nitrate contamination is a real problem in our waterways, and that’s why we’ve had some fish die-offs in previous years,” Smith said. “The last thing we want is to contribute more contaminated water, because our aquatic ecosystem is absolutely critical to the livelihood of South Florida.”

Raising Funds and Future Leaders

The Friends of The Underline are attempting to deliver the project in ways that provide a model for similar green infrastructure installations worldwide. This includes innovative approaches to financing, workforce development, and performance monitoring.

The Underline, expected to entail a roughly USD $156 million price tag when complete, has received ample support from Miami-Dade County as well as the three municipalities — Miami, South Miami, and Coral Gables — connected by the project. It also has benefited from extensive grant support from the State of Miami, the U.S. Department of Transportation, local nonprofits, and other partners. Much of the permitting and design work was completed pro bono by local experts eager to support a highly visible, public-good initiative.

“It’s truly a public–private partnership,” Smith said. “We have federal funding, state funding, local funding, and private dollars all making this work.”

One way The Underline team succeeds in attracting private-sector money is by providing businesses an opportunity to give back to the communities in which they work. For instance, this benefit was a draw for Bank of America (Charlotte, North Carolina), which funded The Underline’s Green Leaders apprenticeship program alongside the U.S. Department of Agriculture and the Daly Family Fund (Miami).

The program accepts five students from Miami Dade College each year as apprentices, who help construct and maintain bioswales, gardens, and other nature-based elements. They also gain technology skills. The Underline maintains a large network of sensors — measuring ecological factors such as temperature, soil and air quality, and runoff flow —generating data that the apprentices help interpret. Additionally, the team is performing drone surveillance of the existing segments of The Underline and is gathering data to feed an “aspirational” digital twin of the linear park that will improve future decision-making, Smith said. Two cohorts of the Green Leaders program already have graduated, with some receiving paid positions with Friends of The Underline. Grant funding will support at least three additional cohorts to come.

“What we have found is that by giving our apprentices this foundation in how to cultivate a garden, how to maintain a garden, and then how to use technology to track its impact, we’re upskilling them to the point that some are moving on,” Smith said. “Some are going on to get a 4-year degree in horticulture, and others are learning that this can be a full-time job. Our goal is to teach the next generation how to be more focused on building Miami’s resilience.”

Learn more about The Underline at its website.

Top image courtesy of James Corner Field Operations/Miami-Dade County Department of Transportation and Public Works/Friends of The Underline