Downtown there is 93,058 sf, up from 67,040 sf in the first quarter this year and 47,002 sf for the fourth quarter last year.

In the case of the Telefonica, affiliates of the Spanish telecommunications company vacated about 20,000 sf in 1001 Brickell Bay Dr., acknowledges an unidentified spokesman for Brickell Bay Management, the building's leasing agent. At 1221 Brickell Ave., Telefonica affiliates vacated about 40,000 sf. The building's leasing agent was unavailable for comment.

Local Telefonica officials would not comment on the company's restructuring efforts and referred all questions to corporate officials in Spain, who did not respond to a request from GlobeSt.com.

"We're not allowed to release information about that," a local Telefonica supervisor identified only as Christina told GlobeSt.com.

This wash of sublease space is posing a number of challenges for property owners, C&W's Campbell says.

"Subleasing space is a very difficult process," he says. "The success of subleasing depends on identifying tenants that are creditworthy and need exactly the amount of space available at that specific time and built out already in a manner that is comparable to the potential subtenant's business use."

It also poses a challenge to tenants that sublease space. "If a subtenant is subleasing from a sub-owner that is experiencing economic difficulties, then the subtenant is exposed to potentially significant hardship should the sub-owner default," Campbell says. "If the subtenant doesn't have a non-disturbance agreement, the master owner can dictate new terms and conditions."