About 70 miles off the San Diego coast there is a Navy-owned island that is a place of mystery to most civilians.
On a clear day, San Diegans can see San Clemente Island off our coast. What many don't know, the southern end is covered with bombing ranges.
U.S. Navy ships practice firing at land targets there and it gets pounded on by attack aircraft dropping bombs.
NBCSanDiego traveled to the island with the FBI and a Navy Explosive Ordnance Disposal Team (EOD) to learn what happens when that ordnance duds and is left lying where it hit, ready to explode.
The firing range is a mock airfield but it’s a mine field for people who work and train there until the area can be cleared.
“Everywhere you see a red flag is going to be an unexploded round that has been marked,” said Navy EOD operator Josh Rich.
FBI bomb technicians and EOD operators showed us the process of building fuses, like on firecrackers, then attaching blasting caps.
The techs used about 80 pounds of C4 for one pile of unexploded bombs before attaching a detonation cord.
Once the fuse and blasting caps were added, everyone in the area was told to get out of the way and take cover.
Flames shot as high as a three-story house and shook us as we stood back more than 200 yards.
Both teams train together so they can deploy to war zones for what they call "Post-Blast Investigations."
FBI Bomb Technician Jim Verdi said they combine expertise in collecting evidence with the Navy's experience dealing with IEDs and UXOs.
"The bottom line is we're working together to keep America safe either overseas or here in the states and we can't do that alone," said Verdi.
Lea Sutton covers stories involving San Diego's military community. Send her your thoughts via Twitter @nbcsandiego or add your comment to our Facebook page.
Find more of her stories in our special military section.