As McCarren Park Pool undergoes a $50MM renovation to become, well, a pool again... the LTV Squad has been taking a look back all week at what it was like during the years it sat there being drained of its former glory. Yep, while you were all enjoying your Pool Parties there was a hidden world underneath the concrete—most notably, a tunnel system starting under the main arch. The LTV Squad was kind enough to let us use some of their photos, but be sure to check out their massive catalog here—and below is some video of what it's like in those underground tunnels.