Covert mountain-bike trail building at Miramar "We are not a playground. We train here. We live here. We are buried here."

The trails are banked and well worn through the terrain

The mountain-bike trail behind Miramar National Cemetery at the west end of the Miramar Marine Corps Air Station was crafted with care and skill. It had a bridge over a low point and has drainage pipes through other dips. It's a roller-coaster ride on well-compacted reddish dirt through the canyons west of where the railroad cuts across the 23,000-acre base.

The trails are also indicative of illegal trespassing onto Marine Corps land, says Capt. Chris Robinson.

"There was a funeral service going on here, a family burying a fallen Marine, and these guys pop up out of a canyon on their bikes, sit, and look at the family and drink out of water bottles before they take off back into the canyon," Robinson says. "It was disrespectful to the Marine and to the family."

The incident, he says, was the tipping point on hard-line enforcement against the band of renegades who have brought a range of tools from shovels and hoes, to some kind of earth-moving machinery, most likely a small Bobcat, to clear and degrade a huge circle near the tracks.

"We are not a playground," Robinson says. "We train here. We live here. We are buried here."

It's hard to tell when the trespassers began working on the trails, but it apparently has been for a long time. The earth is compacted and no plants are springing up in them even after recent rains. The trails are banked and well worn through the terrain, and they are marked by recent tire tracks. From the high points, you can see the businesses on Miramar Road and the Avenue of Flags that leads into the cemetery.

The trails stretch for miles, cutting steeply up and down hills, sharply through switchbacks and spurs between the cemetery and the train tracks. The trespassers have cut fences — including those protecting endangered species — and destroyed the signs telling them they are trespassing on sensitive habitat.

Damaged sign

"We have to replace most of the signs within a month or six weeks of when we put them in," Robinson says. "We put them right on the trail, close enough that you can see the next one while you're standing at this one."

The builders have been working within the past month: green and yellow flags mark a series of switchbacks from the bottom of a canyon to the top, where digging and shaping the trail has begun but ends abruptly after the third tight turn. The land has been dedicated to either the cemetery, which is being built in phases, or as sensitive habitat that's home to vernal pools and endangered native plants or for mitigation.

The cemetery's first phase was dedicated in January 2010 — it is filling up quickly, and work is underway on Phase 2. The cemetery was designed around the vernal pools that are home to fairy shrimp and to San Diego mint, both endangered species.

And then there's the Vortex, a complex of bike trails that someone put a lot of thought and work into. Not only does it put bicyclists in the middle of funerals, it's in the take-off path of the airstrip, with Osprey and Sea Stallion heavy helicopters and FA-18s flying over.

"It's a security problem for so many reasons," Robinson says.

The base military police believe the builders are sneaking onto the base using the fence along Miramar Road and through the utility gates along I-805. And they're watching now, more closely than ever. They want to make a case against the builders that's bigger than just trespassing because the builders are damaging sensitive habitat the Marines have worked hard to establish and nurture.

"There's no way of knowing for sure how many are involved yet," Robinson says. "But it's more than a few. This is a sustained effort over a long period of time." The base's environmental staff has met some of the trespassers, says David Boyer, a biologist on the base who works on habitat. "We always try to educate people," he says. "We've worked hard to make sure that there's been zero loss of habitat and we monitor to make sure what we've done succeeds and thrives.”