Somewhat predating Chernobyl Diaries in the extreme tourism stakes, this has four young visitors to Berlin joining their guide "Dante" (Riemelt), for a trip underground, into the network of tunnels beneath the city. Their target is a recently-discovered Nazi bunker, though it soon becomes apparent they are not the only ones down there. An unfortunate incident cripples one of the party: two of the remainder are sent to get help, leaving Lucia (Kelley) and Denis (Eversman) with the injured party. Help appears to arrive, in the form of tunnel veteran Armin (Stiglmeier), who takes them back to his underground lair, and given them soup and sympathy, having phoned for help. However, as he talks to the pair, it becomes clear that former border-guard Armin is a few signals short of a working train-set. He knocks Denis out, and when he regains consciousness, quickly discovers that no phone-call could possibly have been made, and Lucia is in even worse peril.
After a promising start, introducing a very creepy setting, this starts to fall apart after Armin shows up, and disintegrates steadily through the second half, simply because Denis is a fucking idiot. He's apparently intent on acting out every horror clicé from the past 20 years, and moves at the speed of a treacle glacier, even when in IMMINENT PERIL FROM A MURDEROUS PSYCHOPATH. Lucia isn't much better, and while you initially are rooting for them to escape their situation, by the end, death seems like a suitable payoff for their stupidity. Despite some impressive moments of gore, this results on those being entirely empty from an emotional perspective. The script has too many loose ends: the first pair the group meet seem threatening, but are never seen again, while the final scene poses more questions than it answers. It would have been way cooler if, for instance, Dante had been in league with Armin throughout, and if you can't think of a dozen other ways to improve the script, you probably fell asleep.
And is the film title "Explorers" or "Explorer"? The print says the latter, which would make more sense, since there's clearly more than one of them, but all the promo material and the IMDB entry skips the S. It's kinda symbolic, that the movie is not really certain of its own name.