What you are describing is what is known as the Lacus Curtius (Pool of Curtius), and yes, the pit did exist and is still marked today in the forum however, it may not have been as much of a chasm as is made out by the sources. There are three Roman explanations regarding the Lacus Curtius, which Varro summarises quite nicely for us (On the Latin Language. 6.148-150).

Firstly, he writes that Procilius states that the Earth opened up and upon failure to fill it up, the senate spoke to the oracle who said to fill it up they would have to sacrifice what made them the strongest. With this, a man named Marcus Curtius, rode with his horse into the chasm leading to the closure of the chasm. It is this version of events to which Livy belives is the more truthful (Livy. 7.6.1-6)

Secondly, Varro writes that L. Calpurnius Piso Frugi wrote that in the Sabine War, a Sabine hero named Mettius Curtius ended up in the part of the forum that was still a swamp (Rome was surrounded by marshland) and escaped from the swamp to his men on the Capitoline Hill, which led to the pool being given his name. Livy includes this version also within his work (Livy. 1.12.9, 1.13.5).

And thirdly, according to Cornelius and Lutatius, the place was struck by lightning and by decree of the Senate was fenced in. It was done by the consul Curtius which is why it was called the lacus Curtius.

We have no idea which of the origin stories are true (unless there is Italian scholarship which has touched on the subject). And, whilst the 'pit' does exist - we do not know the extent of it yet. The forum was drained and this may have been an area in which water continued to form (a spring maybe?). If the land was swamp/marsh land prior, it may have been the case of an earthquake displacing water and pit may have appeared. On the other hand, the story of Livy may have just been a myth-story and in fact there was never really a chasm, just a small pit as it is today which would make the last two reasons more likely.