WMG / La Brea

The sinkhole contains an Anomaly.

The characters aren't in an underground Lost World. They traveled through time, and are now actually in the Pleistocene. The sinkhole had a bright, glowing light at the bottom when it opened, just like the Anomalies in Primeval, so it stands to reason that it was part of the same phenomenon, or at least a related one.

• The time travel, at least, is seemingly confirmed due to Gavin finding Eve's necklace after she dropped it in the pilot.

Due to sci-fi similarities...

• The anomaly is the crack in the sky at the end of Futurama's direct to video episode Bender's Big Score and leading into The Beast With a Billion Backs.
  Bender: Well, we're boned.
• The anomaly is the crack in the wall from the Doctor Who episode "The Eleventh Hour".

The past shown in the show is roughly at the end of the Pleistocene and beginning of the Holocene.

Humans didn't arrive in the Americas until the tail-end of the Pleistocene, roughly 12,000-10,000 BC. Around that time was the Younger Dryas Period, the Pleistocene/Holocene transition, when the Ice Age megafauna went extinct. Considering the Clovis/Proto-Native American was able to see the flare in the pilot episode, this puts the 'past' events of the show around the time of the Younger Dryas period.

• It's confirmed they're in about 10,000 BC.

Regarding the indigenous peoples...

Proto-Native American people are introduced in the cliffhanger at the end of the pilot episode. The images in the previews at the end of the pilot suggest the red hand-print image is a cultural motif/totem, and they are heavily implied to practice Human Sacrifice. At the moment, it is currently unknown if they will be friendly or hostile to the time-displaced modern Americans.

• Played with. The "natives" are explicitly depicted as including both proto-Native-Americans and previous groups of time-lost castaways who had Gone Native. What initially looked like sacrificial pits in the teaser turned out to be open-air graves, and the red handprint is a protection totem.

The objects were spread over a wider area than they came from.

From the air, everything that fell into the hole is a good short walk from each other. However the ambulance and other items seem to have dropped far from each other.

Veronica and Lilly's secret

• A very sad one. Veronica is not Lilly's sister but her mother. The religious man whether Veronica’s father or not was a pedophile and Veronica was something of a child bride
  • Ehhh I don't know cuz if he was a pedophile Veronica wouldn't be upset over his death by Savage Wolves.
    • I was wrong but victims don’t always know they’re victims. He was clearly abusive to them in some way and Veronica is still upset.
• I doubt it. Veronica looks to be at most a few years older than Lilly. The youngest Veronica could possibly be for this to happens is around 8 years old.
• It turns out eventually that Lily isn't Veronica's daughter. They're not even related; Lily was kidnapped by the supposed "father" a year before the sinkhole. Whether he'd done the same thing to Veronica and brainwashed her is unstated as of episode 5.

Someone is conducting experiments in time travel, and the sinkholes are the result of it

.The Sinkholes are opening too regularly, and portals are lasting too regularly (opening and then closing five to seven days later) for it to be natural or spontaneous. Someone is trying to create a time machine, and the sinkholes are either the deliberate result or a by-product of tearing holes in time.

• Heavily implied in Episode 8, with the implication that Rebecca is a serial time-traveler, and had met Silas before.

Gavin time-traveled from the past

The sinkholes also work in reverse, and Gavin is originally from 10,000 B.C.

• Confirmed, but played with, in Episode 8: Isaiah (the boy who would grow up to become Gavin) and his grandfather Silas are depicted as living in 10,000 BC, but are said to have fallen through another sinkhole.

In the same vein as the above, the giant snake in Episode 4 isn't native to 10,000 B.C.

That snake was at least the size of an anaconda, and no snake that big lived in Pleistocene North America. Every other prehistoric animal we see in the show is from the correct place and time, so the snake being out of place is jarring to say the least. But what if there's a reason for that? If the sinkholes can work in reverse, animals and people can also travel forward in time through them, potentially leading to animals being found far outside their original time periods. The giant snake wasn't an error— it was foreshadowing.

• Or maybe the snake was some Angelino's actual pet anaconda, and was being transported in one of the vehicles that fell through the L.A. sinkhole along with the people. The owner either avoided falling through or died before telling any of the other castaways about their reptilian buddy, which escaped and survived long enough to get dangerously hungry.
• Potentially confirmed. The eighth episode reveals that a modern-day cow has somehow ended up in 10,000 BC, so the idea of modern animals falling through the sinkholes and becoming established there is now canon.

Awkward family reunion

Silas's grandson Isaiah will grow up to be Gavin Harris. He's been unknowingly interacting with his granddaughter-in-law and great-grandson the whole time, and hasn't discovered it yet. If Silas discovers this, things are going to get awkward.

Silas is Old!Gavin.

Well, we have "Current Gavin" and "Young Gavin", so maybe Silas is old Gavin trying to.. god only knows what.