The darkened gym once frequented by 2,100 ConocoPhillips employees sits locked and empty, its idled workout machines a testament to mounting trouble for commercial office space in Houston.

The workers here left two years ago as plunging oil prices forced downsizing throughout the energy industry. Their neighbors working for BP soon followed. Today, entire floors of Two Westlake Park are deserted, and ConocoPhillips struggles to find a tenant to sublease the space as it continues to pay rent until its own lease ends in 2019.

The pattern is particularly familiar in the westside Energy Corridor, but the damage is not confined there. More than a fifth of Houston office space, about 46.7 million square feet, sits empty - the highest rate among the nation's 20 biggest cities. Vacancy rates have reached levels not seen since the epic 1980s oil bust.

More than 10.5 million square feet of offices remain under lease but are not used by an original tenant. More than 2 million square feet of this sublease space is slated to be returned to property owners in the next two years, according to NAI Partners, portending financial problems for landlords who have loan payments to make.

"Right now, sublease is the tenant's problem," said Leta Wauson, director of research. "Once that lease expires, then the owner has to worry about it."

RELATED: Houston leads the nation in office sublease space

Continued weakness in the office-space market has led Morningstar Credit Ratings to put drastically more commercial property loans on its watch list because of their risk of default. Between 2014 and 2016, Morningstar added about $219 million in loans to its watch list. In the first seven months of 2017, it added more than $1.1 billion.

"So far, Houston has been able to skirt by with a below-average delinquency rate, but we foresee it picking up," said Steve Jellinek, a vice president at Morningstar. The high-and-climbing vacancy rate in Houston office buildings, he said, "has to translate into a higher delinquency rate sooner or later."

The ratings agency on Tuesday announced its latest addition to the watch list, a $91 million loan taken in 2014 at the height of the shale boom against the 17-story Two Westlake Park.

Morningstar's analysis showed that a weak market has pushed down the value of the 450,154-square-foot building - including the gym, a perk to the office workers there - by about 34 percent since the loan was issued. The building value fell to $82 million, down from $124 million, putting the collateral value below the value of the loan.

It's the 51st loan on Morningstar's Houston watch list, which includes loans taken out against 21 office buildings, 12 hotels, eight apartment buildings, seven retail centers, two self-storage facilities and one industrial property.

Kroll Bond Rating Agency put Two Westlake Park atop its list of largest projected losses, estimating the loan will default with more than $30 million in losses. Next on the list is Three Westlake Park, a neighboring building with an $80 million loan for which Kroll forecasts a $27 million loss.

"The main thing we're looking at in Houston is the Energy Corridor," Morningstar's Jellinek said.

On Thursday afternoon, energy and finance company employees milled about the sleepy 1980s-era business park in West Houston. Inside Two Westlake Park, a ground-floor suite sat empty. Vacant offices dot many levels, and the whole sixth floor is tidy and unused. Some levels up, epic images of oil field hardware adorn quiet, empty hallways under spotlights that are turned off.

Pat Hicks, CEO of Houston-based Hicks Ventures, which bought the property in 2014, sounded an optimistic note. He cited Houston's job and population growth as well as positive energy sector earnings reports as indicators that the market would pick up.

"There is no question we are off the bottom," he wrote in an email. "Like the previous four recoveries I've been through, once it really takes hold it'll be better and steeper than anyone thought possible."

Hicks did not respond to requests for a follow-up interview.

RELATED: Oil and gas, manufacturing continue turnaround in Houston

Morningstar has also cited concern for the neighboring Three Westlake Park, which is about $10 million underwater on an $80 million loan with cash flow down by half thanks to vacancy.

If those properties miss loan payments, they will become delinquent. Houston's office market has a 3 percent delinquency rate for office buildings, and that could rise.

In June, Kroll Bond Rating Agency reported a 120 percent increase over 15 months in the number of local energy-related loans in default or facing risk of default.

"Overall, the commercial real estate slide (particularly the office sector) is expected to persist over the next one to two years as office leases for oil companies expire," the ratings group analysis of Houston concluded.

Kroll projects $139.5 million in losses for office properties in Greater Houston.

"We're seeing cracks in the (Houston) office market," said Marc McDevitt, an associate director over commercial mortgage-backed securities at Kroll. "We see more of it coming."

Still, McDevitt said Houston's situation is neither "dire" nor "catastrophic," but rather typical for an energy slump in an oil town.

NAI Partners reports that more than a quarter of Houston's office space, including office space that is sitting vacant, is on the market.

"If it's not vacant this month, it'll be vacant shortly," Wauson said.

FROM 2016: Office market woes to last a while

Commercial real estate tends to lag the general market because large tenants sign long-term leases. Those leases will continue to expire through coming years.

At Two Westlake Park, occupancy fell to 68 percent from 89 percent in March as one of BP North America's two leases expired. ConocoPhillips and BP will also see large leases expire in 2019. Without replacement tenants, occupancy in Two Westlake Park could tumble to 43 percent.

Jellinek cited concern for delinquency risk throughout the Energy Corridor, an area particularly tied to the oil industry. He said that unless oil creeps back above $50 per barrel and big companies start signing big leases again, "it's going to have consequences for these office buildings."

More Information Available Office vacancy rates in the 10 largest U.S. cities: New York: 10.6% Los Angeles: 14.9% Chicago: 15.4% HOUSTON: 22.4% Atlanta: 17.5% Phoenix: 19.6% Philadelphia: 11% SAN ANTONIO: 15% San Diego: 12.4% DALLAS: 19.4% Source: JLL