The end finally comes for “ER” with a one-hour retrospective (7 p.m., NBC) and a two-hour finale (8 p.m.). Not that the last six months haven’t been backward-gazing enough.

To put the longevity and long-toothedness of “ER” into some perspective, the show debuted the same week as “Friends” in September 1994. “NYPD Blue” was just a year old when “ER” bowed. The O.J. trial and attendant media frenzy were still in the future.

“ER” remains that rarest of TV phenomena — the instant hit. Only the presence of “Seinfeld” kept “ER” from being the No. 1 show of its debut season. And it would attain that ultimate ranking in 1995 and cling to it for the rest of the decade.

Like any long-running series, “ER” offered viewers plenty of chances to give up on the show. “ER” seemed to have more than its share of “very special” episodes, overhyped departures and a caravan of disasters that included everything but a zeppelin crash.

Conducting a brief and unscientific survey, I plugged the phrase “I stopped watching ‘ER’ when” into a search engine and came up with hundreds of responses, from the heartfelt to the amusing. A clear plurality saw Dr. Greene’s (Anthony Edwards) death as their bridge too far. One wag wrote, “When they blew up the hospital for the third time.”

You could fill a month’s worth of columns with the names of stars and guest stars whose careers have been enhanced by “ER.” But the biggest star to come out of “ER” is not named Edwards, Clooney, Wyle or Margulies. It’s the Stedicam, or whatever you call the camera technology that allowed every saunter down a hospital corridor to look like a race against death. When “ER” debuted, it looked like no other show on television. It brought a frenetic pace and life-and-death stakes to a large and loyal weekly audience, and it continued to do so for 15 years.

• Not every reality series can spark an international incident. The Odyssey Marine Exploration team, the folks of “Treasure Quest” (9 p.m., Discovery) fame, make a discovery that culminates in every treasure-hunter’s dream and every explorer’s nightmare.

Using Zeus, their remotely operated vehicle, in waters between Spain and Morocco, the Explorer crew discovered and retrieved 17 tons of silver and gold coins scattered over an ocean floor the size of several football fields. Presented in high definition, we see Zeus extracting clumps of coins from the deep ocean floor and collecting them in buckets. Eventually, there are enough Spanish pieces of eight to fill a chartered Boeing 757 — a fortune estimated at $500 million.

The second half of this exciting “Quest” covers the legal wrangle over the remarkable discovery and the Spanish government’s use of police and military vessels to stop Odyssey from undertaking what they see as a plunder of their national legacy and a violation of a sacred historical site.

• Lance Krall returns to “Free Radio” (10 p.m., VH1) as a morning radio show intern whose dim-witted approach attracts a vast audience and a parade of celebrity guests. Tonight: Ed Helms from “The Office.”

Tonight’s other highlights

• Evidence points to murder by meteorite on “Bones” (7 p.m., Fox).

• Murder stalks the Mexican wrestling circuit on “CSI” (8 p.m., CBS).

• A woman blames the FBI for the loss of her child on “Eleventh Hour” (9 p.m., CBS).